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/sandbox-observer-role.yaml

@@ -0,0 +1,23 @@
+apiVersion: rbac.authorization.k8s.io/v1
+kind: Role
+metadata:
+  name: sandbox-observer
+  namespace: poimen-sandbox
+rules:
+  - apiGroups: [""]
+    resources: [pods, pods/log, services]
+    verbs: [get, list, watch]
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: poimen-devs-sandbox
+  namespace: poimen-sandbox
+subjects:
+  - kind: Group
+    apiGroup: rbac.authorization.k8s.io
+    name: "oidc:poimen-devs"
+roleRef:
+  kind: Role
+  name: sandbox-observer
+  apiGroup: rbac.authorization.k8s.io