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/gitrepository.yaml

@@ -0,0 +1,20 @@
+# This repo, as a source Flux can read. It had none: its Kustomizations point at
+# k8s-platform for the templates, and nothing ever needed to read the repo that
+# declares them — because those declarations were applied by hand.
+#
+# Reuses the domain's existing deploy key rather than minting a new one, having
+# verified that key reads the dataplane repositories. Worth knowing what that
+# implies: the key is not scoped to a single repository, so the isolation its
+# per-repo name suggests does not exist.
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: dataplane-sales-dev
+  namespace: sales
+spec:
+  interval: 5m
+  url: ssh://git@gitea.sttlab.eu:2222/stt/kong-dataplane-sales.git
+  secretRef:
+    name: sales-api-git-auth
+  ref:
+    branch: main