fix(flow): 完成分支合并逻辑修复

修复内容：
1. branch.py: _merge_normal 使用 merge_squash 替代错误的 reset_soft
2. tracker.py: finish 阶段先提交再合并，合并前额外提交状态文件
3. branch.py: 添加 _cleanup_lock_file 方法清理 lock 文件

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

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

.aide/branches.json

@@ -65,7 +65,9 @@
       "task_id": "2025-12-17T06-03-45",
       "task_summary": "最终测试：验证合并功能",
       "started_at": "2025-12-17T06:03:45+08:00",
-      "status": "active"
+      "status": "finished",
+      "end_commit": "d60f0bae2d783625d628aabf91fa74f631053a7c",
+      "finished_at": "2025-12-17T06:05:19+08:00"
     }
   ]
 }