Multi-agent pub/sub coordination for AI agents working in parallel. Use when: (1) Project has .hydra/ directory initialized, (2) Working with multiple agents or parallel tasks, (3) Need to coordinate, communicate, or share state with other agents, (4) Starting a session and should check what other agents did, (5) Finishing work and should notify other agents. Triggers: multi-agent, coordinate, other agents, parallel agents, emit, broadcast, hydra-mail, check messages, agent coordination.