Notification Channels (EW-663)

Generic INotificationChannelPlugin contract covering Discord, Slack, Telegram, WhatsApp and Novu (meta-router) with fan-out + retry + per-channel delivery log.

• spec.md — feature specification
• plan.md — implementation plan
• tasks.md — task breakdown

Sibling specs in the notifications-v2 umbrella: email-providers, event-subscriptions, agent-inbox-ui.