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/src/platform.rs

@@ -0,0 +1,37 @@
+use std::path::Path;
+
+use crate::GitToolingError;
+
+#[cfg(unix)]
+pub fn create_symlink(
+    _source: &Path,
+    link_target: &Path,
+    destination: &Path,
+) -> Result<(), GitToolingError> {
+    use std::os::unix::fs::symlink;
+
+    symlink(link_target, destination)?;
+    Ok(())
+}
+
+#[cfg(windows)]
+pub fn create_symlink(
+    source: &Path,
+    link_target: &Path,
+    destination: &Path,
+) -> Result<(), GitToolingError> {
+    use std::os::windows::fs::FileTypeExt;
+    use std::os::windows::fs::symlink_dir;
+    use std::os::windows::fs::symlink_file;
+
+    let metadata = std::fs::symlink_metadata(source)?;
+    if metadata.file_type().is_symlink_dir() {
+        symlink_dir(link_target, destination)?;
+    } else {
+        symlink_file(link_target, destination)?;
+    }
+    Ok(())
+}
+
+#[cfg(not(any(unix, windows)))]
+compile_error!("codex-git-tooling symlink support is only implemented for Unix and Windows");