[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.

首先，將以資料夾為基礎的工作階段檔案樹狀瀏覽器與以儲存庫為基礎的工作階段的Changes檢視進行比較，並追蹤側面板的進入點。加入Files檢視，使其能夠瀏覽並開啟worktree中的檔案，同時保留Changes；接著驗證使用乾淨儲存庫的重現能夠顯示並開啟完整的樹狀結構。

由索引模型根據 Issue 內容生成。