Updated system skills bundled with Codex were not correctly replacing
the user's skills in their .system folder.
- Fix `.codex-system-skills.marker` not updating by hashing embedded
system skills recursively (nested dirs + file contents), so updates
trigger a reinstall.
- Added a build Cargo hook to rerun if there are changes in
`src/skills/assets/samples/*`, ensuring embedded skill updates rebuild
correctly under caching.
- Add a small unit test to ensure nested entries are included in the
fingerprint.
