Add none option to status_comments for merge queue

The status_comments option in the merge_queue configuration now supports a none value, which disables all merge queue status comments on pull requests.

This provides users with three levels of control over queue notifications:

• all (default): Post comments for all queue events, including entering the queue, CI progress updates, and final outcomes
• outcomes: Only post comments for final outcomes (merged or dequeued with failure reason)
• none: Do not post any comments for queue status Example configuration:

merge_queue:
  status_comments: none

This is useful for repositories that want to reduce notification noise and rely solely on the merge queue dashboard or other monitoring tools for queue status updates.