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.toml

@@ -9,6 +9,7 @@ members = [
     "exec",
     "execpolicy",
     "file-search",
+    "git-tooling",
     "linux-sandbox",
     "login",
     "mcp-client",
@@ -39,6 +40,7 @@ codex-common = { path = "common" }
 codex-core = { path = "core" }
 codex-exec = { path = "exec" }
 codex-file-search = { path = "file-search" }
+codex-git-tooling = { path = "git-tooling" }
 codex-linux-sandbox = { path = "linux-sandbox" }
 codex-login = { path = "login" }
 codex-mcp-client = { path = "mcp-client" }