## Summary
- Fixed the pr-review-responder workflow failing to find PRs from forks
- Added a fallback that searches for open PRs using `pulls.list` API
with the head repository owner and branch name when
`workflow_run.pull_requests` array is empty

## Test plan
- Create a PR from a fork with the `cc:request` label
- Verify the workflow can now correctly find and process the PR

#skip-bugbot

Generated with [Claude Code](https://claude.com/claude-code)


<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes PR lookup in the pr-review-responder workflow for forked PRs. Adds
a fallback search by head owner and branch when
workflow_run.pull_requests is empty, so fork PRs are processed
correctly.

- **Bug Fixes**
  - Handle empty workflow_run.pull_requests for cross-repo PRs.
  - Use pulls.list with head "<owner>:<branch>" to find the PR number.
  - Exit early (should_continue=false) if no matching open PR is found.

<sup>Written for commit 70e3c1399b438d5fe768aa9171ec2bf683bc38ff.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

---------
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>