Declarative commit_message_format for merge commits

A new commit_message_format option lets you control merge commit content with a simple declarative config: pick inherit or pr-title for the title, inherit, pr-body, or empty for the body, and opt into trailers like co-authored-by, approved-by, or merged-by. Use inherit to defer to GitHub’s repository-level “default commit message” setting. Available on queue rules, the merge action, and defaults. The existing commit_message_template keeps working unchanged.