Email Providers (EW-650)

Tenant-managed inbound/outbound email + per-Agent assignment + multi-provider plugins, with React-Email composer integration and first-class agent-to-agent email (v1.1).

• spec.md — feature specification (v1.1)
• plan.md — implementation plan
• tasks.md — task breakdown

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