Deleting a session in the app doesn't remove it from session-store.db / VS Code Copilot Chat history (orphaned session)

### Describe the bug

Deleting a session from the Copilot app UI does not propagate the deletion to the shared ~/.copilot session store. The session row remains fully intact in data.db, session-store.db (including search_index), and vscode.session.metadata.cache.json. Because the VS Code Copilot extension reads this same shared store, the "deleted" session keeps appearing in VS Code's Copilot Chat history panel.

This appears to be the same underlying "delete doesn't prune session-store.db" root cause reported in #3811 (which surfaces via /chronicle insights), just with a different symptom (VS Code history panel). As I'm experiencing it in the App I made it a different bug.

### Affected version

GitHub Copilot CLI 1.0.69

### Steps to reproduce the behavior

1. Create a new session using the "New session" button on a repo.
2. Set it to local (not worktree) and ask it anything.
3. The session now appears in both the Copilot app and the VS Code Copilot Chat history pane.
4. Delete the session from the app.
5. It remains in the VS Code chat history pane, with full chat history persisting after restarts even.

### Expected behavior

Deleting a session in the app should also remove it from the VS Code Copilot Chat history pane, so it no longer appears (in either the app or VS Code) after deletion.

### Additional context

Operating system: Windows

アプリのセッション削除フローから開始し、それが data.db、session-store.db とその search_index、および vscode.session.metadata.cache.json をどのように更新するかを追跡します。ローカルセッションで再現し、削除によって保存された行が削除されること、さらに両方のアプリを再起動した後にセッションが VS Code Copilot Chat の履歴に表示されなくなることを確認します。

索引モデルが issue の本文から書いたものです。