#2751 [spike] review-bot test, do not merge

[spike] Throwaway test PR for the code-review-bot spike. Do not merge, enqueue, or review by hand. It exists only so the spike's review bot has a PR to post a real review on. It will be closed when the spike is done.

What changes

A GitHub-event automation can now fire when a pull request is merged (pull-request-merged): the Luke GitHub App's pull_request closed delivery is read as a merge, and the run is told the pull request's number, title, address, and branch, like the other pull request events.

🤖 Generated with Claude Code

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Summary by cubic

This is a throwaway test PR for the review-bot spike and should not be merged. It adds a pull-request-merged automation trigger so the spike's review bot has a real merge event to review.

• Reads a GitHub pull request closed delivery as a merge, passing the pull request's number, title, address, and base branch to the run like the other pull request events.
• The branch filter reads the base the pull request merged into.
• A close without a merge, or one made by a bot, fires nothing so an automation cannot fire itself.