onGitHubPullRequestMerged classifies GitHub’s "closed" delivery when pull_request.merged is true.
automations/watch-merged-github-pull-requests.automation.ts
account defaults to the project binding. Returns Signal<GitHubPullRequestMergedEvent> with action: "closed", pull_request.merged: true, and the complete typed payload. An unmerged close emits the separate closed trigger.