新增: 会话分叉功能，优化增量扫描缓存

- 新增 c 键触发会话分叉，支持带方向提示的分叉输入
- 使用 claude --fork-session 原生分叉，不污染原会话
- Init() 启动时自动触发历史扫描，无需手动切 tab
- 缓存提升到 HistoryState 内存持有，避免重复 I/O
- 新增项目目录 modTime 增量扫描，未变目录跳过遍历
- 扫描后裁剪缓存，删除磁盘已不存在的条目
- updateSummaryInCache 直接操作内存缓存

internal/app.go

@@ -35,6 +35,8 @@ type Model struct {
 	update      UpdateState
 	wsFocus     int // workspace 焦点列: 0=tabs 1=list
 	wsTabCur   int // tabs 列光标 (0..len(Groups), 最后一个是历史入口)
+	forkMode    bool   // 分叉输入模式
+	forkBuf     string // 分叉方向提示输入
 }
 
 func NewModel() *Model {
@@ -46,7 +48,11 @@ func NewModel() *Model {
 
 func (m *Model) Init() tea.Cmd {
 	m.update.Checking = true
-	return CheckUpdateCmd()
+	cmds := []tea.Cmd{CheckUpdateCmd()}
+	if cmd := m.onTabSwitch(); cmd != nil {
+		cmds = append(cmds, cmd)
+	}
+	return tea.Batch(cmds...)
 }
 
 func (m *Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
@@ -58,11 +64,23 @@ func (m *Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		m.height = msg.Height
 	case ScanCompleteMsg:
 		m.history.Projects = msg.Projects
+		m.history.Cache = msg.Cache
 		m.history.Loaded = true
 		m.history.Scanning = false
 		if m.history.Favorites == nil {
 			m.history.Favorites = loadFavorites()
 		}
+		// 填充 ForkFrom
+		forks := loadForks()
+		if len(forks) > 0 {
+			for _, pd := range m.history.Projects {
+				for _, s := range pd.Sessions {
+					if src, ok := forks[s.ID]; ok {
+						s.ForkFrom = src
+					}
+				}
+			}
+		}
 		total := 0
 		for _, pd := range msg.Projects {
 			total += len(pd.Sessions)
@@ -134,11 +152,11 @@ func (m *Model) handleKey(s string) (*Model, tea.Cmd) {
 }
 
 func (m *Model) onTabSwitch() tea.Cmd {
-	if m.IsHistoryTab() && !m.history.Loaded && !m.history.Scanning {
+	if !m.IsHistoryTab() || m.history.Scanning {
-		m.history.Scanning = true
+		return nil
-		return ScanSessionsCmd()
 	}
-	return nil
+	m.history.Scanning = true
+	return ScanSessionsCmd()
 }
 
 // --- Workspace 按键 ---
@@ -206,6 +224,11 @@ func (m *Model) moveGroupCursor(dir int) {
 // --- HISTORY 按键 ---
 
 func (m *Model) handleHistoryKey(s string) (*Model, tea.Cmd) {
+	// fork 输入模式拦截
+	if m.forkMode {
+		return m.handleForkInput(s)
+	}
+
 	switch s {
 	case "up", "k":
 		if m.history.FocusPanel == 0 {
@@ -243,10 +266,37 @@ func (m *Model) handleHistoryKey(s string) (*Model, tea.Cmd) {
 	case "n":
 		return m.newSessionFromHistory()
 	case "r", "f5":
-		m.history.Scanning = true
+		if cmd := m.onTabSwitch(); cmd != nil {
-		return m, ScanSessionsCmd()
+			return m, cmd
+		}
 	case "f":
 		m.toggleFavorite()
+	case "c":
+		if m.currentSession() != nil && m.history.FocusPanel == 1 {
+			m.forkMode = true
+			m.forkBuf = ""
+		}
+	}
+	return m, nil
+}
+
+// handleForkInput 处理分叉方向提示输入
+func (m *Model) handleForkInput(s string) (*Model, tea.Cmd) {
+	switch s {
+	case "enter":
+		return m.executeFork()
+	case "esc":
+		m.forkMode = false
+		m.forkBuf = ""
+	case "backspace":
+		if len(m.forkBuf) > 0 {
+			m.forkBuf = m.forkBuf[:len(m.forkBuf)-1]
+		}
+	default:
+		// 只接受可打印字符
+		if len(s) == 1 && s[0] >= 32 && s[0] < 127 {
+			m.forkBuf += s
+		}
 	}
 	return m, nil
 }
@@ -365,6 +415,57 @@ func (m *Model) newSessionFromHistory() (*Model, tea.Cmd) {
 	return m, nil
 }
 
+func (m *Model) executeFork() (*Model, tea.Cmd) {
+	s := m.currentSession()
+	if s == nil {
+		m.forkMode = false
+		return m, nil
+	}
+	m.forkMode = false
+
+	cwd := s.Cwd
+	if isWindows {
+		cwd = strings.ReplaceAll(cwd, "/", "\\")
+	}
+	title := s.CustomTitle
+	if title == "" {
+		title = s.ID[:8]
+	}
+	label := "fork: " + title
+
+	// 构建 claude -r <id> --fork-session [prompt]
+	if m.forkBuf != "" {
+		// 用户输入了方向提示
+		prompt := m.forkBuf
+		m.forkBuf = ""
+		if isWindows {
+			escaped := strings.ReplaceAll(prompt, `"`, "`\"")
+			script := fmt.Sprintf(`$Host.UI.RawUI.WindowTitle = "fork: %s"
+		Write-Host "=== Fork from %s ===" -ForegroundColor Cyan
+		cd "%s"
+		claude -r %s --fork-session "%s" --permission-mode bypassPermissions`, title, title, cwd, s.ID, escaped)
+			launchInWtWithTitle(cwd, label, script)
+		} else {
+			escaped := strings.ReplaceAll(prompt, "'", "'\\''")
+			m.pendingCmd = fmt.Sprintf(`printf '\033]0;fork: %s\007' && echo "=== Fork from %s ===" && cd "%s" && claude -r %s --fork-session '%s' --permission-mode bypassPermissions`, title, title, cwd, s.ID, escaped)
+		}
+	} else {
+		// 纯分叉，不带方向提示
+		if isWindows {
+			script := fmt.Sprintf(`$Host.UI.RawUI.WindowTitle = "fork: %s"
+		Write-Host "=== Fork from %s ===" -ForegroundColor Cyan
+		cd "%s"
+		claude -r %s --fork-session --permission-mode bypassPermissions`, title, title, cwd, s.ID)
+			launchInWtWithTitle(cwd, label, script)
+		} else {
+			m.pendingCmd = fmt.Sprintf(`printf '\033]0;fork: %s\007' && echo "=== Fork from %s ===" && cd "%s" && claude -r %s --fork-session --permission-mode bypassPermissions`, title, title, cwd, s.ID)
+		}
+	}
+
+	m.launched = label
+	return m, nil
+}
+
 // --- AI 摘要 ---
 
 func (m *Model) applySummary(sessionID, summary string, completed, pending []string) {
@@ -377,7 +478,7 @@ func (m *Model) applySummary(sessionID, summary string, completed, pending []str
 				s.AISummary = summary
 				s.Completed = completed
 				s.Pending = pending
-				saveSummaryToCache(sessionID, summary, completed, pending)
+				updateSummaryInCache(m.history.Cache, sessionID, summary, completed, pending)
 				return
 			}
 		}
@@ -542,6 +643,9 @@ func (m *Model) render() string {
 	if m.inputBuf != "" {
 		b.WriteString(style.InputStyle.Render(fmt.Sprintf(" ▶ num:%s [Enter]go [Esc]cancel", m.inputBuf)))
 	}
+	if m.forkMode {
+		b.WriteString(style.InputStyle.Render(fmt.Sprintf(" ⎇ fork:%s [Enter]confirm [Esc]cancel", m.forkBuf)))
+	}
 	if m.launched != "" {
 		b.WriteString(lipgloss.NewStyle().Foreground(style.Accent).Render(fmt.Sprintf(" ✓ %s", m.launched)))
 		m.launched = ""
@@ -585,6 +689,7 @@ func (m *Model) renderHelp() []string {
 			m.fmtHelp("Enter", "resume"),
 			m.fmtHelp("f", "star"),
 			m.fmtHelp("n", "new"),
+			m.fmtHelp("c", "fork"),
 			m.fmtHelp("1", "workspace"),
 			m.fmtHelp("r/F5", "refresh"),
 			m.fmtHelp("q", "quit"),
@@ -1012,6 +1117,14 @@ func (m *Model) detailColumnLines(w, listH int) []string {
 
 	lines = append(lines, lipgloss.NewStyle().Foreground(style.Dim).Render(fitWidth("  [Enter]resume", w)))
 
+	if s.ForkFrom != "" {
+		forkID := s.ForkFrom
+		if len(forkID) > 8 {
+			forkID = forkID[:8]
+		}
+		lines = append(lines, lipgloss.NewStyle().Foreground(style.Cyan).Render(fitWidth(fmt.Sprintf("  ⎇ fork from %s", forkID), w)))
+	}
+
 	if len(lines) > listH {
 		lines = lines[:listH]
 	}

internal/history.go

@@ -30,6 +30,7 @@ type Session struct {
 	Completed   []string  // AI 提取的已完成项
 	Pending     []string  // AI 提取的待办项
 	FilePath    string    // JSONL 文件完整路径
+	ForkFrom    string    // 分叉来源 session ID（空表示非分叉）
 }
 
 // ProjectDir 按目录分组的会话集合
@@ -89,6 +90,7 @@ type HistoryState struct {
 	Loaded     bool
 	Scanning   bool
 	Favorites  map[string]bool
+	Cache      map[string]*cacheEntry // 内存缓存，避免重复 I/O
 }
 
 // IsHistoryTab 判断当前是否在 HISTORY Tab
@@ -100,36 +102,53 @@ func (m *Model) IsHistoryTab() bool {
 
 type ScanCompleteMsg struct {
 	Projects   []*ProjectDir
-	UpdatedIDs map[string]bool // modTime 变化的会话，需要重新生成摘要
+	Cache      map[string]*cacheEntry // 更新后的缓存
+	UpdatedIDs map[string]bool        // modTime 变化的会话，需要重新生成摘要
 }
 
 func ScanSessionsCmd() tea.Cmd {
 	return func() tea.Msg {
-		projects, updated := scanAllProjects()
+		projects, cache, updated := scanAllProjects()
-		return ScanCompleteMsg{Projects: projects, UpdatedIDs: updated}
+		return ScanCompleteMsg{Projects: projects, Cache: cache, UpdatedIDs: updated}
 	}
 }
 
-func scanAllProjects() ([]*ProjectDir, map[string]bool) {
+func scanAllProjects() ([]*ProjectDir, map[string]*cacheEntry, map[string]bool) {
 	home, err := os.UserHomeDir()
 	if err != nil {
-		return nil, nil
+		return nil, nil, nil
 	}
 	projectsDir := filepath.Join(home, ".claude", "projects")
 	entries, err := os.ReadDir(projectsDir)
 	if err != nil {
-		return nil, nil
+		return nil, nil, nil
 	}
 
-	cache := loadCache(home)
+	cache, projTimes := loadCacheWithTimes(home)
 	dirMap := make(map[string]*ProjectDir)
 	updatedIDs := make(map[string]bool)
+	seenIDs := make(map[string]bool)
 
 	for _, entry := range entries {
 		if !entry.IsDir() {
 			continue
 		}
 		projectPath := filepath.Join(projectsDir, entry.Name())
+		info, err := entry.Info()
+		if err != nil {
+			continue
+		}
+
+		// 项目目录 modTime 未变 → 从缓存直接重建该目录下的会话
+		if prev, ok := projTimes[projectPath]; ok && prev.Equal(info.ModTime()) {
+			for _, id := range rebuildFromCache(cache, dirMap, projectPath) {
+				seenIDs[id] = true
+			}
+			continue
+		}
+		// 目录有变更，记录新 modTime
+		projTimes[projectPath] = info.ModTime()
+
 		files, err := os.ReadDir(projectPath)
 		if err != nil {
 			continue
@@ -139,13 +158,14 @@ func scanAllProjects() ([]*ProjectDir, map[string]bool) {
 				continue
 			}
 			sessionID := strings.TrimSuffix(f.Name(), ".jsonl")
+			seenIDs[sessionID] = true
 			filePath := filepath.Join(projectPath, f.Name())
-			info, err := f.Info()
+			finfo, err := f.Info()
 			if err != nil {
 				continue
 			}
 
-			if cached, ok := cache[sessionID]; ok && cached.ModTime.Equal(info.ModTime()) {
+			if cached, ok := cache[sessionID]; ok && cached.ModTime.Equal(finfo.ModTime()) {
 				addSessionToDir(dirMap, cached.ToSession(sessionID))
 				continue
 			}
@@ -160,11 +180,31 @@ func scanAllProjects() ([]*ProjectDir, map[string]bool) {
 			session.Pending = nil
 			updatedIDs[sessionID] = true
 			addSessionToDir(dirMap, session)
-			cache[sessionID] = cacheEntryFrom(session, info.ModTime())
+			cache[sessionID] = cacheEntryFrom(session, finfo.ModTime())
 		}
 	}
 
-	saveCache(home, cache)
+	// 裁剪缓存：删除磁盘已不存在的条目
+	for id := range cache {
+		if !seenIDs[id] {
+			delete(cache, id)
+		}
+	}
+
+	// 清理已不存在项目的 projTimes
+	activeProjects := make(map[string]bool, len(entries))
+	for _, e := range entries {
+		if e.IsDir() {
+			activeProjects[filepath.Join(projectsDir, e.Name())] = true
+		}
+	}
+	for p := range projTimes {
+		if !activeProjects[p] {
+			delete(projTimes, p)
+		}
+	}
+
+	saveCacheWithTimes(home, cache, projTimes)
 
 	result := make([]*ProjectDir, 0, len(dirMap))
 	for _, pd := range dirMap {
@@ -176,7 +216,20 @@ func scanAllProjects() ([]*ProjectDir, map[string]bool) {
 	sort.Slice(result, func(i, j int) bool {
 		return result[i].Dir < result[j].Dir
 	})
-	return result, updatedIDs
+	return result, cache, updatedIDs
+}
+
+// rebuildFromCache 从缓存重建指定项目目录下的所有会话，返回匹配的 session ID
+func rebuildFromCache(cache map[string]*cacheEntry, dirMap map[string]*ProjectDir, projectPath string) []string {
+	prefix := filepath.Base(projectPath)
+	var seen []string
+	for id, e := range cache {
+		if filepath.Base(filepath.Dir(e.FilePath)) == prefix {
+			addSessionToDir(dirMap, e.ToSession(id))
+			seen = append(seen, id)
+		}
+	}
+	return seen
 }
 
 func addSessionToDir(dirMap map[string]*ProjectDir, s *Session) {
@@ -360,8 +413,9 @@ func findStringEnd(data []byte) int {
 const cacheVersion = 2
 
 type cacheFile struct {
-	Version  int                      `json:"version"`
+	Version      int                      `json:"version"`
-	Sessions map[string]*cacheEntry   `json:"sessions"`
+	Sessions     map[string]*cacheEntry   `json:"sessions"`
+	ProjectTimes map[string]time.Time     `json:"projectTimes"` // 项目目录 modTime
 }
 
 type cacheEntry struct {
@@ -377,6 +431,7 @@ type cacheEntry struct {
 	Completed   []string  `json:"completed"`
 	Pending     []string  `json:"pending"`
 	FilePath    string    `json:"filePath"`
+	ForkFrom    string    `json:"forkFrom"`
 }
 
 func (e *cacheEntry) ToSession(id string) *Session {
@@ -386,7 +441,7 @@ func (e *cacheEntry) ToSession(id string) *Session {
 		MsgCount: e.MsgCount, FirstMsg: e.FirstMsg,
 		AwaySummary: e.AwaySummary, AISummary: e.AISummary,
 		Completed: e.Completed, Pending: e.Pending,
-		FilePath: e.FilePath,
+		FilePath: e.FilePath, ForkFrom: e.ForkFrom,
 	}
 }
 
@@ -397,7 +452,7 @@ func cacheEntryFrom(s *Session, modTime time.Time) *cacheEntry {
 		MsgCount: s.MsgCount, FirstMsg: s.FirstMsg,
 		AwaySummary: s.AwaySummary, AISummary: s.AISummary,
 		Completed: s.Completed, Pending: s.Pending,
-		FilePath: s.FilePath,
+		FilePath: s.FilePath, ForkFrom: s.ForkFrom,
 	}
 }
 
@@ -406,19 +461,32 @@ func cachePath(home string) string {
 }
 
 func loadCache(home string) map[string]*cacheEntry {
+	c, _ := loadCacheWithTimes(home)
+	return c
+}
+
+func loadCacheWithTimes(home string) (map[string]*cacheEntry, map[string]time.Time) {
 	data, err := os.ReadFile(cachePath(home))
 	if err != nil {
-		return make(map[string]*cacheEntry)
+		return make(map[string]*cacheEntry), make(map[string]time.Time)
 	}
 	var cf cacheFile
 	if json.Unmarshal(data, &cf) != nil || cf.Version != cacheVersion {
-		return make(map[string]*cacheEntry)
+		return make(map[string]*cacheEntry), make(map[string]time.Time)
 	}
-	return cf.Sessions
+	projTimes := cf.ProjectTimes
+	if projTimes == nil {
+		projTimes = make(map[string]time.Time)
+	}
+	return cf.Sessions, projTimes
 }
 
 func saveCache(home string, m map[string]*cacheEntry) {
-	cf := cacheFile{Version: cacheVersion, Sessions: m}
+	saveCacheWithTimes(home, m, nil)
+}
+
+func saveCacheWithTimes(home string, m map[string]*cacheEntry, projTimes map[string]time.Time) {
+	cf := cacheFile{Version: cacheVersion, Sessions: m, ProjectTimes: projTimes}
 	data, err := json.Marshal(cf)
 	if err != nil {
 		return
@@ -536,12 +604,10 @@ func extractMessagesForSummary(path string) string {
 	return strings.Join(msgs, "\n")
 }
 
-func saveSummaryToCache(sessionID, summary string, completed, pending []string) {
+func updateSummaryInCache(cache map[string]*cacheEntry, sessionID, summary string, completed, pending []string) {
-	home, err := os.UserHomeDir()
+	if cache == nil {
-	if err != nil {
 		return
 	}
-	cache := loadCache(home)
 	entry, ok := cache[sessionID]
 	if !ok {
 		return
@@ -549,6 +615,11 @@ func saveSummaryToCache(sessionID, summary string, completed, pending []string)
 	entry.AISummary = summary
 	entry.Completed = completed
 	entry.Pending = pending
+	// 异步持久化
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return
+	}
 	saveCache(home, cache)
 }
 
@@ -624,3 +695,41 @@ func collectFavorites(projects []*ProjectDir, favs map[string]bool) []*Session {
 	}
 	return out
 }
+
+// --- Fork 记录 ---
+
+func forksPath(home string) string {
+	return filepath.Join(home, ".u-tabs", "forks.json")
+}
+
+func loadForks() map[string]string {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return make(map[string]string)
+	}
+	data, err := os.ReadFile(forksPath(home))
+	if err != nil {
+		return make(map[string]string)
+	}
+	var m map[string]string
+	if json.Unmarshal(data, &m) != nil {
+		return make(map[string]string)
+	}
+	return m
+}
+
+func recordFork(targetID, sourceID string) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return
+	}
+	forks := loadForks()
+	forks[targetID] = sourceID
+	data, err := json.Marshal(forks)
+	if err != nil {
+		return
+	}
+	dir := filepath.Join(home, ".u-tabs")
+	os.MkdirAll(dir, 0o755)
+	os.WriteFile(forksPath(home), data, 0o644)
+}