feat: poimen-sandbox namespace + security hardening + RBAC

Phase 1: sandbox namespace with PSA restricted enforcement,
ResourceQuota (20 pods, 10/20 CPU, 20/40Gi memory), and
CiliumNetworkPolicy denying pi-session egress to kube-apiserver.

Phase 2: sandbox-observer Role + poimen-devs RoleBinding for
OIDC-authenticated kubectl access to sandbox pod status/logs.

k8s/infra/rbac/kustomization.yaml

@@ -16,6 +16,7 @@ resources:
   - temporal-operator-role.yaml
   - llm-serving-operator-role.yaml
   - poimen-memory-operator-role.yaml
+  - sandbox-observer-role.yaml
 # paperless's Role/RoleBinding lives in k8s/apps/paperless/rbac.yaml instead -
 # that app already has its own kustomization + namespace, no need to
 # duplicate it here. All of these stay inert (grant nothing) until