Review context collection crashes on broken untracked symlinks

Summary

When /codex:review collects working-tree context, it reads untracked files directly from disk.
If the repository contains a broken symlink or an unreadable untracked path, review context collection can throw
and abort the whole review.

Steps to reproduce

1. Create a git repo with a clean initial commit.
2. Add an untracked broken symlink, for example:

   ln -s missing-target broken-link
   
   

3. Run the review flow that collects working-tree context.

Actual behavior

The command crashes while trying to stat/read the untracked path.

Expected behavior

Broken symlinks or unreadable untracked files should be skipped and noted in the review context, instead of
failing the entire review.

Notes

The failure happens in untracked file handling during working-tree context collection.
A resilient fallback would keep the review running and mark the path as skipped.

Start at the untracked file handling in working-tree context collection and reproduce the review flow with a broken symlink or unreadable untracked path. Trace how the path is statted or read, then verify that the review continues, records the path as skipped, and does not crash.