Reliable cleanup of expired Tuist Preview environments

Expired Tuist Preview environments are now cleaned up reliably. The scheduled GitHub workflow that tears down preview namespaces is restored after a tool-installation failure left stale previews running, and an in-cluster Kubernetes CronJob (preview-janitor) now runs hourly as a backup cleanup loop. The janitor deletes any preview namespace whose expiration label has passed and also removes stale “creating” namespaces, so resources no longer linger when external workflows fail.