docs(notifications): add notification system design plans
Imported the Continuum-era notification specs (durable inbox + websocket
foundation, APNs relay, FCM relay, outbound webhooks) and amended them for
Silo. Amendments from the 2026-06-11 review: wire contracts normalized to
Silo naming, back-catalog seeding and per-series burst suppression, durable
dispatch outbox, cross-library episode dedupe, forward-sync wake API,
ticket-based websocket handshake, relay threat-model additions (egress IP,
keyed collapse IDs), and webhook 4xx/SSRF hardening. Includes a
self-contained design-decisions.html visual overview.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
