Remove dead github_notifications code path

[JuanVqz]

Context

Coverband runtime coverage (recording since 2026-04-09) shows 0% runtime on the entire github_notifications code path. Investigation confirmed this is dead code, fully replaced by POST /api/results.

What to remove

• app/controllers/api/github_notifications_controller.rb
• app/services/github_notifications/process.rb
• app/models/github_notification.rb
• Route resources :github_notifications, only: :create in config/routes.rb
• Related tests

Why it's dead

The old flow expected GitHub to send check_run webhooks to POST /api/github_notifications. The current flow has the checker post results directly to POST /api/results with API key auth. No webhook is configured to hit the github_notifications endpoint.

Notes

• The github_notifications DB table can be dropped in a follow-up migration once we confirm no useful historical data needs preserving.
• The route, controller, service, and model have zero production hits over 5 days of Coverband tracking.