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/git-tooling/README.md

@@ -0,0 +1,20 @@
+# codex-git-tooling
+
+Helpers for interacting with git.
+
+```rust,no_run
+use std::path::Path;
+
+use codex_git_tooling::{create_ghost_commit, restore_ghost_commit, CreateGhostCommitOptions};
+
+let repo = Path::new("/path/to/repo");
+
+// Capture the current working tree as an unreferenced commit.
+let ghost = create_ghost_commit(&CreateGhostCommitOptions::new(repo))?;
+
+// Later, undo back to that state.
+restore_ghost_commit(repo, &ghost)?;
+```
+
+Pass a custom message with `.message("…")` or force-include ignored files with
+`.force_include(["ignored.log".into()])`.