fix(flow): 修复 finish 阶段分支合并的多个问题

1. 在 BranchManager 添加 _cleanup_lock_file 方法，清理 lock 文件
2. 将分支合并移到 git 提交之后执行
3. 在分支合并前再次提交状态文件（解决 save_status 更新 hash 后未提交的问题）

测试任务：创建 test-cache/hello.py

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

aide-program/aide/flow/tracker.py

@@ -108,14 +108,6 @@ class FlowTracker:
                 if action == "next-part":
                     assert to_phase is not None
                     validator.validate_next_part(current_phase, to_phase)
-
-                    # 如果进入 finish 环节，执行分支合并
-                    if to_phase == "finish":
-                        success, merge_msg = self.branch_mgr.finish_branch_merge(
-                            task_summary=normalized_text,
-                        )
-                        if not success:
-                            output.warn(merge_msg)
                 elif action == "back-part":
                     assert to_phase is not None
                     validator.validate_back_part(current_phase, to_phase)
@@ -135,6 +127,18 @@ class FlowTracker:
                 final_status = self._do_git_commit(updated, commit_msg)
                 self.storage.save_status(final_status)
 
+                # 如果进入 finish 环节，执行分支合并（必须在提交后执行）
+                if action == "next-part" and to_phase == "finish":
+                    # 再次提交状态文件（因为 save_status 更新了 git_commit hash）
+                    self.git.add_all()
+                    self.git.commit("[aide] finish: 更新状态文件")
+
+                    success, merge_msg = self.branch_mgr.finish_branch_merge(
+                        task_summary=normalized_text,
+                    )
+                    if not success:
+                        output.warn(merge_msg)
+
                 if action == "next-part":
                     output.ok(f"进入环节: {to_phase}")
                 elif action == "back-part":