Add NotificationDestination + config.notifications.destinations and a
`destinations` reference list on toggles/rules. When a toggle/rule references
destinations, delivery resolves from the shared destination; when empty, the
existing inline-field delivery path runs UNCHANGED (zero regression). Lets
email/webhook/mesh-channel be defined once and reused, de-duplicating the
delivery config. UI to follow (C2).
Co-authored-by: Matt Johnson <mj@k7zvx.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
