Point the moved Kustomizations at k8s-platform across namespaces

Moving them into the catalog namespace broke their source: sourceRef
resolves in the Kustomization's own namespace, so "k8s-platform" no longer
matched anything and both reported GitRepository not found.

Name the namespace explicitly rather than duplicating the source per
domain. Cross-namespace refs are allowed here — kustomize-controller runs
without --no-cross-namespace-refs — and the split is the point: the
templates belong to the platform in flux-system, the Kustomization that
consumes them belongs to the team in its own namespace.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

dev/api-gateway/gateway-kustomization.yaml

@@ -13,6 +13,11 @@ spec:
   sourceRef:
     kind: GitRepository
     name: k8s-platform
+    # k8s-platform lives in flux-system: the templates are the platform's,
+    # this Kustomization is the team's. sourceRef resolves in the
+    # Kustomization's own namespace unless told otherwise, hence the
+    # explicit namespace rather than a copy of the source per domain.
+    namespace: flux-system
   path: ./dataplane-template
   prune: true
   postBuild: