Narrow the pull_request trigger to opened/reopened/ready_for_review so
per-commit pushes to open PRs no longer storm the runners via a
rebase-of-all-PRs (PR_FILTER: all) on every synchronize event. Pin
PR_READY_STATE to "all" so draft PRs remain in scope for updates
triggered by push (main advancing) and opened.
Closes#262
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>