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/poimen/poimen-sandbox-namespace.yaml

@@ -0,0 +1,9 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: poimen-sandbox
+  labels:
+    pod-security.kubernetes.io/enforce: restricted
+    pod-security.kubernetes.io/enforce-version: latest
+    pod-security.kubernetes.io/audit: restricted
+    pod-security.kubernetes.io/warn: restricted