Simplify CLA gate workflow

[clockwork-labs-bot]

Summary

• keep the CLA Gate workflow using cargo ci cla-assistant status for CLA Assistant lookups
• stop mirroring pull request/status license/cla results into a custom CLA Gate commit status
• use a plain pull_request trigger for PR checks, and keep merge-group status publishing for synthetic queue SHAs
• let the Actions job pass when license/cla is success and fail when it is missing or non-success

Notes

Merge-group handling is left as-is. This PR intentionally keeps the helper command and only changes the workflow behavior needed to avoid the custom status mirror on PR/status events.

Testing

• cargo fmt --all
• cargo check -p ci
• cargo ci self-docs --check
• Ruby YAML parse
• git diff --check

actionlint unavailable locally.

[bfops]

LGTM