[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.

Commencez par comparer le navigateur de l’arborescence de fichiers de session adossée à un dossier avec la vue Changes de la session adossée au dépôt, puis suivez le point d’entrée du panneau latéral. Ajoutez une vue Files permettant de parcourir et d’ouvrir les fichiers dans le worktree tout en préservant Changes, puis vérifiez que la reproduction avec un dépôt propre affiche et ouvre l’arborescence complète.

Rédigé par le modèle d'indexation à partir du texte de l'issue.