Optimized `scripts/review_pull_requests.py` by replacing multiple `git branch` and `git log` calls with a single `git for-each-ref` command using the `%(ahead-behind:main)` atom (Git 2.41+).
This reduces the number of subprocess calls from O(N) to O(1), significantly improving performance in repositories with many branches.
Also improved error handling for missing `gh` CLI.
