Add CLA gate status wrapper

[clockwork-labs-bot]

Summary

• Add a CLA Gate workflow that publishes a repository-owned commit status.
• Mirror CLA Assistant license/cla status on pull requests and status events.
• Publish CLA Gate=success on merge_group runs, because entries have already passed PR checks before entering the queue.

Required settings change after merge

• Remove license/cla from required checks.
• Add CLA Gate as the required CLA check.

This keeps CLA enforcement before merge queue while removing CLA Assistant from the merge-group critical path.

Test plan

• Parsed .github/workflows/cla-gate.yml with Ruby YAML loader.
• Ran git diff --check.