Add merge group workflow trigger recipe

[Wanbinyu]

What's changed?

• Add org.openrewrite.github.AddMergeGroupTrigger.
• Apply it only to GitHub Actions workflows with a pull_request trigger.
• Cover positive, negative, idempotency, and file-path cases.
• Regenerate recipes.csv.

What's your motivation?

Workflows must subscribe to merge_group for required checks to run in a GitHub merge queue.

• Fixes Add merge_group build trigger #114

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've run ./gradlew build locally, and committed resulting changes to recipes.csv
• I've formatted the lines I changed, without reformatting code I didn't touch