docs(spec): notifications design (engine in IIOS + Web Push, per-thread mute)
Approved design: IIOS notification engine (PresenceService, NotificationProjector with
policy/presence/mute gates, swappable NotificationPort → Web Push VAPID, subscription +
participant.muted data) + host-app experience (service worker, registerPush, mute toggle,
deep-link). Trigger policy: DMs always, group only on @mention or reply-to-you. Phased:
frontend quick win → IIOS engine → SDK/app.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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