Refresh/keep session in sync with external modifications (e.g. via VS Code)

### Feature summary

_No response_

### What problem are you trying to solve?

After implementing something in the app, it's common to e.g. open VS Code to tweak/review/continue manual work. The Github App session is visible inside VS Code, and can be continued there, but the changes aren't picked up back in the Github App; the latter must be restarted for it to reread the session continuation done outside in VS Code.

### Proposed solution

In an ideal world, the App would simply detect external session and show them; since that's probably complicated (two UIs working on the same copilot session), at least some way to refresh the session , similar to what a restart does would help.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

Note slight similarity with #1205, though that issue is more about picking up **new** sessions, whereas here it's about picking up continuations of existing/known sessions.

该 issue 未指明任何文件、测试或入口点。首先跟踪 GitHub App 如何读取 Copilot 会话，以及在 VS Code 中创建的延续如何在应用重启后变得可见；当现有会话可以被刷新以显示外部更改，且无需重启应用时，即表示完成。

由索引模型根据 Issue 内容生成。