Github Webhook

Receive GitHub webhook events.

Handles:

• push: Trigger pull when repo is updated, and tell the context check about it
• pull_request: Reconcile linked Dot proposals, and tell the context check about the head
• check_suite, check_run: Tell the context check about a new head or a Re-run of Dot's check
• installation: Handle app install/uninstall and permission changes
• installation_repositories: Handle repo access changes

The context check only records hints here (Redis, never GitHub); its own lane does the work. Each hint runs in a thread: recording it in Redis and enqueuing a cycle on the broker block, and a stalled Redis would otherwise hold every request this worker serves.