Add a trigger for pull request head updates

[thaning0]

Related discussion: getpaseo/paseo#3924

Note

This is a draft for feedback on the workflow, event semantics, and public event name. A companion public-docs PR will follow once those points are agreed.

Hub workflows can subscribe to the proposed github.pull_request_updated event when GitHub delivers pull_request.synchronize. Other pull request actions do not match this semantic event, and existing GitHub trigger behavior remains unchanged.

Goals

• Expose github.pull_request_updated as a semantic GitHub trigger.
• Classify only pull_request.synchronize deliveries as pull request head updates.
• Preserve pull request item metadata for reaction targeting.
• Summarize updated pull requests in project activity.
• Add regression coverage at the matcher, provider, and activity-summary boundaries.

Non-goals

• Treat all pull request mutations as head updates.
• Add triggers for other pull request actions.
• Change workflow configuration syntax or filtering behavior.
• Add webhook subscriptions, GitHub App permissions, or database migrations.

Why

The raw github.pull_request source is already routed to the GitHub trigger provider. The semantic classifier handles opened and labeled pull requests, but does not represent the synchronize action used for pull request head updates.

This change adds that missing semantic classification without changing the underlying webhook boundary.

Documentation

The companion public-docs PR is pending agreement on the event semantics and name in the linked discussion.

Verification

• Targeted Vitest coverage for GitHub matching, provider behavior, and activity summaries: 3 files and 56 tests passed.
• npm run release:check: passed.
• Full repository integration checks will run in GitHub Actions.

Risk surface

Limited to GitHub pull request semantic classification and the corresponding activity summary. Regression tests verify that synchronize matches the proposed event while metadata edits do not.