Imports the notification system design folder (architecture overview,
release-events/inbox foundation, APNs/FCM relay specs, outbound webhooks)
and adds the Web Push spec (05, implemented in this branch), the shared
outbound-email architecture note, and the v1.5 roadmap (06) covering the
remaining work after APNs/FCM were deferred to v2.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
