feat: git tooling for undo (#3914)

## Summary
Introduces a “ghost commit” workflow that snapshots the tree without
touching refs.
1. git commit-tree writes an unreferenced commit object from the current
index, optionally pointing to the current HEAD as its parent.
2. We then stash that commit id and use git restore --source <ghost> to
roll the worktree (and index) back to the recorded snapshot later on.

## Details
- Ghost commits live only as loose objects—we never update branches or
tags—so the repo history stays untouched while still giving us a full
tree snapshot.
- Force-included paths let us stage otherwise ignored files before
capturing the tree.
- Restoration rehydrates both tracked and force-included files while
leaving untracked/ignored files alone.

codex-rs/tui/src/chatwidget/tests.rs

@@ -335,6 +335,8 @@ fn make_chatwidget_manual() -> (
         suppress_session_configured_redraw: false,
         pending_notification: None,
         is_review_mode: false,
+        ghost_snapshots: Vec::new(),
+        ghost_snapshots_disabled: false,
     };
     (widget, rx, op_rx)
 }