Add agent status tracking (status, status_updated_at)

Add `status` and `status_updated_at` columns to the agents table so agents can report their availability state. ## Changes Required 1. **Migration:** Add columns to agents table - `status` integer, default: 0 (idle) - `status_updated_at` datetime 2. **Model updates:** - Add `enum status: { idle: 0, busy: 1 }` - Add scope `idle` - Add scope `busy` - Add instance method `idle_for?(duration)` - returns true if status is idle and status_updated_at is older than duration 3. **No React Callbacks:** AgentChannel does NOT update status on connect/disconnect (WebSocket is transient) ## Deliverable Agents can track and report their own state via MCP tools (separate task). ## Parent Epic #62 - Autonomous Agent Coordination via WebSocket