Add email channel support

[owainlewis]

Goal

Let push receive and reply through email for lower-urgency assistant workflows.

Why

Email is broadly useful, but it has different expectations around latency, threading, formatting, and safety.

Scope

• Evaluate IMAP/SMTP versus provider-specific APIs.
• Define allowlist and thread mapping for email senders and subjects.
• Decide how much quoted history to send to the agent.
• Send plain-text replies safely.

Acceptance criteria

• A documented design picks the first email transport.
• Allowlisted emails can trigger an agent reply.
• Reply threading works for common clients.
• Attachments and HTML are either handled safely or explicitly out of scope.