fix: skip broken untracked symlinks in review context

[zylar06]

Summary

• skip broken or unreadable untracked paths during working-tree review context collection
• keep review context generation from crashing on broken untracked symlinks
• add a regression test covering broken untracked symlinks

Closes #65

Testing

• node --test tests/git.test.mjs

[stoneHee99]

Heads up — my PR #48 also modifies formatUntrackedFile in git.mjs (adds a stat.isDirectory() guard for issue #14). Since both PRs touch the same function, there will likely be a merge conflict depending on which one lands first. Just flagging so the maintainers can coordinate. 👍