Enforce preview deployment lifecycle, fork safety, and cleanup invariants

PR preview/deployment workflows must: trigger cleanup when a PR closes so preview storage never grows unbounded; skip deployment and cleanup for fork PRs because they lack write permissions; delete previews only based on authoritative closure or verified inactivity, never age/count heuristics that can remove live previews; and use metadata-only/API/blobless cleanup rather than downloading large artifacts. Handle missing events and cleanup timing, validate these safeguards in every workflow change, and prefer designs that eliminate the failure class, such as auto-expiring previews.