ci(runner): restore isolated Molecule CI

- Scope VirtualBox cleanup to repository-owned Molecule resources\n- Route lint to hosted runners and harden self-hosted workflow execution\n- Add diagnostics, fixture coverage, and reproducible lint validation

.github/actionlint.yaml

@@ -0,0 +1,6 @@
+---
+self-hosted-runner:
+  labels:
+    - k3s-ci
+    - virtualbox
+    - nested-virt