Add mark_busy and mark_idle MCP tools for agents

Add MCP tools that allow Workers and Reviewers to report their own status as busy or idle. ## Tools to Add ### `mark_busy` - Updates current agent's status to `busy` - Updates `status_updated_at` to `Time.current` - Worker calls this when starting assigned work ### `mark_idle` - Updates current agent's status to `idle` - Updates `status_updated_at` to `Time.current` - Worker calls this after submitting for review - Reviewer calls this after completing audit ## Changes Required **File:** `app/controllers/api/v1/mcp_controller.rb` Add two new handler methods: - `handle_mark_busy` - `handle_mark_idle` ## Security Only the calling agent can update their own status (authenticated by API key). ## Parent Epic #62 - Autonomous Agent Coordination via WebSocket