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/Cargo.toml

@@ -35,6 +35,7 @@ codex-common = { workspace = true, features = [
 ] }
 codex-core = { workspace = true }
 codex-file-search = { workspace = true }
+codex-git-tooling = { workspace = true }
 codex-login = { workspace = true }
 codex-ollama = { workspace = true }
 codex-protocol = { workspace = true }