Update PR workflow

Write the full replacement pull request title and the complete replacement PR body so they reflect the current scope, motivation, and validation. Run update-pr with the title as the first argument and the body on stdin. Finally, report the pull request URL.

bunx @willbooster/agent-skills@latest update-pr "feat: concise PR title" <<'EOF'
Close #<issue>

## Summary

- What changed
- Key scope or affected area
- Any notable tradeoff or limitation

## Why