[Feature Request] Copilot App: Full file-tree browser for repository-backed sessions (like folder sessions have)

### Summary

When opening a **folder-backed session**, the app shows a full file-tree browser in the side panel, so I can navigate and open any file in the directory directly. When opening a **repository-backed session** (worktree), the side panel instead shows only a git **Changes** view, which lists *modified* files. With a clean working tree it just says "No files found" / "No changes to compare," and there's no way to browse the repo's files within the app.

### Why this matters

The folder-session file browser is a much nicer integration — it lets me explore and open files without leaving the app or asking the agent. Repository sessions lose that, even though all the files are present on disk in the worktree.

### Request

Add a full file-tree browser to repository-backed sessions (the same explorer that folder sessions already have), ideally alongside the existing git **Changes** view rather than replacing it — e.g. a "Files" tab next to "Changes."

### Repro

1. Open a repository as a session (creates a worktree).
2. With no edits made, look at the side panel.
3. The Changes tab shows "No files found" — there's no way to browse the repo's existing files.

### Expected

Be able to browse and open any file in the repo/worktree from within the app, like folder-backed sessions allow.

まず、フォルダーを基盤とするセッションのファイルツリーブラウザーと、リポジトリを基盤とするセッションのChangesビューを比較し、サイドパネルのエントリーポイントを追跡します。Changesを維持したままworktree内のファイルを参照して開けるFilesビューを追加し、その後、クリーンなリポジトリでの再現ケースでツリー全体が表示され、開け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。