refactor: make this a reusable deployment, not ours

Every environment value is a ${VAR} for the caller to substitute: hostname,
images, storage, ingress class, TLS issuer, Vault server and path, timezone,
retention. Nothing in k8s/ names a host, a realm, an agent or an estate.

Removed: the Flux plumbing under dev/, which described one deployment, and the
ExternalSecret pulling a specific agent's Keycloak client secret. Both belong to
whoever deploys this. The lab CA ConfigMap goes with them — the mount stays,
parameterised and marked optional, because 'trust a private CA' is a property of
an environment and not of n8n.

The README is now the template's: how to consume it, what every variable means,
and the three things it deliberately does not do.

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

k8s/n8n/pvc.yaml

@@ -5,9 +5,9 @@ apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
   name: n8n-data
-  namespace: n8n
+  namespace: ${NAMESPACE}
 spec:
   accessModes: ["ReadWriteOnce"]
   resources:
     requests:
-      storage: 2Gi
+      storage: ${N8N_STORAGE}