Copilot CLI loses todos and edits the wrong plan after forked

### Describe the bug

If Copilot is asked to update the plan and the todo list, it will update the old plan in the forked session and reports the todo list being empty.

This is likely a problem not only for the plan mode, but whenever it needs to edit a file in the session folder.

### Affected version

1.0.78

### Steps to reproduce the behavior

1. Ask the agent to create a plan with todos
2. Fork the session
3. Ask it to update the plan and todos

### Expected behavior

The agent should edit the plan file and the todo list under the new session folder. The todo list should be copied over for the new session

### Additional context

_No response_

通过创建包含待办事项的计划、fork 会话，并要求 agent 更新两者来重现该问题。跟踪 fork 后的会话如何在新的会话文件夹下选择其计划文件和待办事项列表，然后验证待办事项列表已被复制，并且两个文件是在 fork 后的会话中而不是原始会话中更新的。

由索引模型根据 Issue 内容生成。