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/Cargo.lock

@@ -778,6 +778,16 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "codex-git-tooling"
+version = "0.0.0"
+dependencies = [
+ "pretty_assertions",
+ "tempfile",
+ "thiserror 2.0.16",
+ "walkdir",
+]
+
 [[package]]
 name = "codex-linux-sandbox"
 version = "0.0.0"
@@ -919,6 +929,7 @@ dependencies = [
  "codex-common",
  "codex-core",
  "codex-file-search",
+ "codex-git-tooling",
  "codex-login",
  "codex-ollama",
  "codex-protocol",