Add agent commands for self-reporting state:
- bd agent state <agent> <state>: Update agent state and last_activity
- bd agent heartbeat <agent>: Update last_activity timestamp only
- bd agent show <agent>: Display agent bead details
States: idle, spawning, running, working, stuck, done, stopped, dead
Also adds AgentState and LastActivity fields to UpdateArgs in RPC protocol.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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