github-code-review worker: fix Ctrl-C shutdown, partial checkout cleanup, base branch fallback

- Add global $running checks in checkoutBranch retry loop - Ctrl-C now exits immediately during checkout
- Clean up partial checkout directory before retry to avoid 'unexpected disconnect'
- Add getPRBaseBranch() to look up actual base branch via GitHub API when stored branch is wrong
- Add $running checks throughout processJob at all blocking I/O points
- Handle 'shutdown' return in main() loop to break immediately
- Extract review prompts to scripts/prompts/review.txt for easy tweaking
- New --review-bots / -b flag to include bot PRs in review (default: skip)
- Non-blocking dequeue when waitIfEmpty=false (was blocking with timeout=0)

Author: detain