check -> build -> mirror, self-hosted runner, buildah to localhost:5000, k3s
smoke test. Follows the gitea-ci skill template and its act_runner gotchas
(secrets inlined in run:, no heredocs). check will be RED until the engine is
implemented (acceptance suite). Deploy job omitted until k3s manifests exist in
infra. GH_DEPLOY_KEY secret must be set before mirror succeeds.
