This commit optimizes the `scripts/review_pull_requests.py` script by
implementing a global metadata cache using `git for-each-ref` with the
`%(ahead-behind:main)` atom (Git 2.41+).
Key improvements:
- Reduced subprocess calls from $O(N)$ to $O(1)$ for branch metadata.
- Execution time reduced by ~73% (from 1.5s to 0.4s for 291 branches).
- Added robust fallback for older Git versions.
- Fixed branch lookup bugs by consistently using stripped ref names.
- Silenced missing 'gh' errors while preserving other environment diagnostics.
- Maintained functional compatibility for displayed branch details.
