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/ingress/ingress.yaml

@@ -2,24 +2,24 @@ apiVersion: networking.k8s.io/v1
 kind: Ingress
 metadata:
   name: n8n
-  namespace: n8n
+  namespace: ${NAMESPACE}
   annotations:
     # The editor pushes execution progress over a long-lived connection, and
     # imports can be large. Buffering it defeats the first; the default body
     # size defeats the second.
-    nginx.ingress.kubernetes.io/proxy-body-size: "50m"
+    nginx.ingress.kubernetes.io/proxy-body-size: ${INGRESS_BODY_SIZE}
     nginx.ingress.kubernetes.io/proxy-read-timeout: "3600"
     nginx.ingress.kubernetes.io/proxy-send-timeout: "3600"
     nginx.ingress.kubernetes.io/proxy-buffering: "off"
     nginx.ingress.kubernetes.io/proxy-http-version: "1.1"
 spec:
-  ingressClassName: nginx
+  ingressClassName: ${INGRESS_CLASS}
   tls:
     - hosts:
-        - n8n.sttlab.local
+        - ${N8N_HOSTNAME}
       secretName: n8n-tls
   rules:
-    - host: n8n.sttlab.local
+    - host: ${N8N_HOSTNAME}
       http:
         paths:
           - path: /