Rename Backstage ClusterRole to avoid Helm conflict

Renamed from backstage-environment-manager to backstage-k8s-plugin
to avoid being overwritten by the Backstage Helm chart.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

environment-provisioner/backstage-rbac.yaml

@@ -3,7 +3,7 @@
 apiVersion: rbac.authorization.k8s.io/v1
 kind: ClusterRole
 metadata:
-  name: backstage-environment-manager
+  name: backstage-k8s-plugin
 rules:
   # Namespace management for environment lifecycle
   - apiGroups: [""]
@@ -55,11 +55,11 @@ rules:
 apiVersion: rbac.authorization.k8s.io/v1
 kind: ClusterRoleBinding
 metadata:
-  name: backstage-environment-manager-binding
+  name: backstage-k8s-plugin-binding
 roleRef:
   apiGroup: rbac.authorization.k8s.io
   kind: ClusterRole
-  name: backstage-environment-manager
+  name: backstage-k8s-plugin
 subjects:
   - kind: ServiceAccount
     name: backstage