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/src/lib.rs

@@ -570,7 +570,6 @@ mod tests {
         // Create a unique CODEX_HOME per test to isolate auth.json writes.
         let codex_home = get_next_codex_home();
         std::fs::create_dir_all(&codex_home).expect("create unique CODEX_HOME");
-
         Config::load_from_base_config_with_overrides(
             ConfigToml::default(),
             ConfigOverrides::default(),