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/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+name = "codex-git-tooling"
+version.workspace = true
+edition.workspace = true
+readme = "README.md"
+
+[lib]
+name = "codex_git_tooling"
+path = "src/lib.rs"
+
+[dependencies]
+tempfile = "3"
+thiserror = "2"
+walkdir = "2"
+
+[lints]
+workspace = true
+
+[dev-dependencies]
+pretty_assertions = "1.4.1"