The notification now owns its payload via #payload method in Pushable,
allowing direct access like notification.payload.title. Push classes
simply use the notification's payload rather than building it themselves.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Separates notification payload construction from push delivery by
introducing DefaultPayload, EventPayload, and MentionPayload classes
that encapsulate the title, body, and URL generation for each
notification type.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>