新增: Git worktree生命周期管理(创建/提交/合并/冲突检测/清理)

- create_plan_worktree: 创建Plan级工作分支

- create_worktree: 创建SubTask专属worktree(天然文件隔离)

- commit_worktree: worktree内提交改动(含user.email兜底)

- merge_branch: merge-tree预检+merge实际合并,返回Clean/Conflict

- remove_plan_worktrees: 批量清理Plan所有worktree+分支

- 7个集成测试全绿(隔离/并行/提交/无冲突合并/冲突检测/非Git降级/清理)

- 使用真实Git+tempdir测试(参考df-execute/tests/shell.rs模式)

crates/df-ai/Cargo.toml

@@ -18,3 +18,6 @@ reqwest = { version = "0.12", features = ["stream", "json"] }
 futures = "0.3"
 eventsource-stream = "0.2"
 rand = "0.8"
+
+[dev-dependencies]
+tempfile = "3"