feat(relay): add agent-to-agent messaging system

[bborn]

Summary

• Implement Agent Relay-style messaging for agent-to-agent communication
• Add relay_messages table for persistent message storage
• Create internal/relay package with Relay manager, agent registry, and message routing
• Integrate with executor for agent registration on task start
• Add CLI commands: ty relay send/read/list
• Display relay messages in task detail logs with 📨 icon

Implementation Details

Database Schema:

• New relay_messages table with id, from_agent, to_agent, content, task_id, status, timestamps
• Indexed by to_agent and status for efficient queries

Relay Package (internal/relay/):

• Relay struct manages agents and messages in memory with DB persistence
• Support for direct messages, broadcast (*), and case-insensitive agent names
• DBStore adapter connects relay to database layer

Executor Integration:

• Agents registered when tasks start (name derived from task title)
• Idle detection tracks last output time per task
• Messages delivered when agents are idle (~1.5s no output)

CLI Commands:

• ty relay send <to> <message> - Send message to agent
• ty relay read [id] - Read message by ID or list for agent
• ty relay list - List connected agents

UI:

• Relay messages displayed with 📨 icon in task detail logs

Test plan

• All existing tests pass
• New relay package tests pass (8 test cases)
• Manual test: Send message from CLI, verify stored in DB
• Manual test: Verify agents registered when tasks start processing
• Manual test: Verify relay list shows processing/blocked tasks

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