Multi-agent message claiming (lease/ack model)

[longevityboris]

Problem

Multiple agents sharing a domain can race on the same inbound email. No claim/lease/ack mechanism exists — two agents can both read and reply to the same message.

Proposal

1. Add claimed_by and claimed_at fields to messages table
2. Add inbox claim <id> --agent <name> command with optimistic locking
3. Add inbox ack <id> and inbox fail <id> for completion tracking
4. inbox ls --unclaimed filter for agents to find work
5. Optional TTL on claims (auto-release after N minutes if not acked)

Priority: HIGH — required for multi-agent setups