fix: switch ingress to GCE native class

NGINX Ingress Controller was not deployed; use GKE's built-in GCE
ingress which provisions a GCP HTTP Load Balancer automatically.

Remove /grafana path: GCE ingress does not support ExternalName
backends. Grafana remains accessible via kubectl port-forward.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>