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 to catalog. The id no longer transits a shared namespace
either, which was a compromise this repo documented rather than liked.

Adds the GitRepository this repo never had — its Kustomizations point at
k8s-platform for templates, and nothing needed to read the repo that
declares them, because those declarations were applied by hand. It reuses
the domain's existing deploy key rather than minting one; verified that
the key reads this repository. Worth knowing what that implies: the key is
not scoped to a single repo, so the isolation its per-repo name suggests
does not exist.

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

Migration is not a patch: a namespace is immutable, so the existing
objects have to be deleted and recreated. Deleting a Kustomization with
prune enabled would take its Gateway, GatewayConfiguration and mirror CR
with it, so prune has to be turned off on the old objects first, then the
new ones adopt what is already running.

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

dev/api-gateway/gitrepository.yaml

@@ -0,0 +1,21 @@
+# 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. Verified
+# that it reads this repository: a throwaway GitRepository with secretRef
+# catalog-api-git-auth reached Ready against this URL. 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-catalog-dev
+  namespace: catalog
+spec:
+  interval: 5m
+  url: ssh://git@gitea.sttlab.eu:2222/stt/kong-dataplane-catalog.git
+  secretRef:
+    name: catalog-api-git-auth
+  ref:
+    branch: main