Add in-session undo/rewind capability (like CLI's /undo / VSCode checkpoints)

Thank you for sharing an idea. Please describe the workflow and the problem you want the GitHub App to solve.

### Feature summary
Add in-session undo/rewind to roll back conversation history and file changes to a previous prompt, similar to Copilot CLI's `/undo` / double-Esc rewind and VSCode's checkpoint restore.

### What problem are you trying to solve?
The GitHub App desktop currently has no way to undo changes or rewind conversation history within a session. If an agent makes unwanted modifications ? or if I simply want to revisit an earlier approach without losing context ? the only option is to discard the entire session and start over, which wastes time, tokens, and prior useful context.

Both Copilot CLI (`/undo`, `/rewind`, double-Esc rewind picker) and VSCode (checkpoints with Restore Checkpoint) have this capability. The desktop app, being built on Copilot CLI, is the only surface missing it. This creates a noticeable gap for users who rely on the desktop app for agent-driven development.

### Proposed solution
Introduce an undo/rewind mechanism in the desktop app session. Ideally:
- A way to view recent prompts as rewind points in the conversation timeline (similar to CLI's rewind picker).
- Selecting a point rewinds the conversation and optionally restores files to the state at that prompt.
- Visual indication (e.g. a button or right-click option on previous messages) to rewind to here, similar to VSCode's checkpoint hover behavior.

Since the app uses isolated git worktrees per session, the underlying infrastructure (snapshots per prompt, git-based rewind) could reuse the same mechanism Copilot CLI already implements.

### Workflow impact
All desktop app users would benefit. This is especially impactful for:
- Users running long agent sessions where mistakes happen midway and restarting is costly.
- Users coming from CLI or VSCode who expect comparable session control in the desktop app.
- Teams adopting the desktop app as their primary Copilot surface.

### Installation context
Individual install (Copilot Pro / Pro+).

### Additional context
- CLI rewind docs: https://docs.github.com/en/copilot/how-tos/copilot-cli/roll-back-changes
- VSCode checkpoints docs: https://code.visualstudio.com/docs/chat/chat-checkpoints
- The app already has session-level isolation (git worktrees per session) ? prompt-level snapshots would be a natural extension.

No implementation files, tests, or entry points are identified in the issue. Start by reviewing the linked Copilot CLI rollback and VSCode checkpoint documentation, then trace the desktop app's existing session and worktree handling. Done means users can select a prior prompt, rewind conversation state, and optionally restore files without discarding the session.

Written by the indexing model from the issue text.