新增: 会话分叉功能，优化增量扫描缓存

- 会话分叉: 按 c 键从历史会话分叉，支持带方向提示
- 启动自动加载历史记录
- 增量扫描: 缓存内存化、目录 modTime 跳过、已删除条目裁剪
- 刷新按钮复用 onTabSwitch 单一入口

internal/history.go

@@ -733,3 +733,51 @@ func recordFork(targetID, sourceID string) {
 	os.MkdirAll(dir, 0o755)
 	os.WriteFile(forksPath(home), data, 0o644)
 }
+
+// --- AI 摘要与收藏操作 ---
+
+func (m *Model) applySummary(sessionID, summary string, completed, pending []string) {
+	if summary == "" {
+		return
+	}
+	for _, pd := range m.history.Projects {
+		for _, s := range pd.Sessions {
+			if s.ID == sessionID {
+				s.AISummary = summary
+				s.Completed = completed
+				s.Pending = pending
+				updateSummaryInCache(m.history.Cache, sessionID, summary, completed, pending)
+				return
+			}
+		}
+	}
+}
+
+func (m *Model) nextSummaryCmd() tea.Cmd {
+	for _, pd := range m.history.Projects {
+		for _, s := range pd.Sessions {
+			needSummary := s.AISummary == "" && s.FirstMsg != ""
+			needDetail := s.AISummary != "" && len(s.Completed) == 0 && len(s.Pending) == 0
+			if (needSummary || needDetail) && s.FilePath != "" {
+				return generateSummaryCmd(s.FilePath, s.ID)
+			}
+		}
+	}
+	return nil
+}
+
+func (m *Model) toggleFavorite() {
+	if m.history.Favorites == nil {
+		m.history.Favorites = loadFavorites()
+	}
+	s := m.currentSession()
+	if s == nil {
+		return
+	}
+	if m.history.Favorites[s.ID] {
+		delete(m.history.Favorites, s.ID)
+	} else {
+		m.history.Favorites[s.ID] = true
+	}
+	saveFavorites(m.history.Favorites)
+}