Add idempotency to the merge-upstream job:
1. Check for existing open PR from chore/upstream-sync-* branch
2. If PR exists and branch is up to date with upstream → skip
3. If PR exists but upstream has new commits → checkout existing
branch, merge new upstream commits, run Claude to review/test
4. If no PR exists → create new branch (existing behavior)
Also update Claude's prompt to handle both create and update PR
cases, so it knows to push to the existing PR branch instead of
creating a duplicate.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
