Move the gateway entrypoints into the domain namespace

The gateway and policies Kustomizations sat in flux-system, and so did the
Secret carrying the control-plane id. Neither was a governance decision:
postBuild.substituteFrom resolves only in the Kustomization's own
namespace, dataplane-template-defaults existed only in flux-system, so
everything that needed it had to live there too — in a namespace the
feature team has no access to. The team could not touch its own gateway.

k8s-platform now places that ConfigMap in every domain namespace, so the
constraint is gone. Move both Kustomizations and the control-plane
ExternalSecret into the domain namespace; the id no longer transits a
shared namespace either.

sourceRef gets an explicit namespace: flux-system, because it resolves in
the Kustomization's own namespace and k8s-platform is not there. Naming it
beats duplicating the source per domain, and the split is the point —
templates belong to the platform, the Kustomization consuming them belongs
to the team. Verified on catalog first: without this the moved objects
report "GitRepository k8s-platform not found".

Adds the GitRepository this repo never had, reusing the domain's existing
deploy key; that key was verified to read the dataplane repositories,
which also means it is not scoped to one repo.

And the bootstrap Kustomization watching ./dev/api-gateway, so all of it
reconciles from git from now on.

Migration is not a patch: a namespace is immutable, so these objects are
deleted and recreated. Deleting a Kustomization with prune enabled would
take its Gateway, GatewayConfiguration and mirror CR with it, so prune is
turned off on the old objects first and the new ones adopt what is already
running. Done that way on catalog with no pod restart and no Kong entity
lost.

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

dev/api-gateway/policies-kustomization.yaml

@@ -7,12 +7,17 @@ apiVersion: kustomize.toolkit.fluxcd.io/v1
 kind: Kustomization
 metadata:
   name: payments-dev-api-gateway-policies
-  namespace: flux-system
+  namespace: payments
 spec:
   interval: 10m
   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-globalpolicy-template
   prune: true
   postBuild: