agentd daemon mode with pi adapter

Core pi integration for agentd:

1. agentd becomes a long-running daemon (vs current run-and-exit) 2. sqlite state for agent registry, run history 3. pi adapter implementing:

• spawn(worktree, config) → agent_id
• send_task(agent_id, prompt) → task_id
• get_status(agent_id) → status
• stop(agent_id)

4. worktree management (git worktree add/remove) 5. webhook callback on completion (reuse pi extension from t-538)

This is the MVP - get pi working reliably under agentd control.