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/certificate.yaml

@@ -1,17 +1,16 @@
-# cert-manager issues this into the `n8n-tls` Secret, from the same
-# ClusterIssuer as the rest of the estate.
+# cert-manager issues this into the `n8n-tls` Secret.
 #
-# Deliberately not a copy of a certificate committed to git: the private key
-# stays in the cluster, is rotated by cert-manager, and never reaches a repo.
+# Deliberately not a certificate copied into the repo: the private key is
+# generated in the cluster, rotated by cert-manager, and never reaches git.
 apiVersion: cert-manager.io/v1
 kind: Certificate
 metadata:
   name: n8n-tls
-  namespace: n8n
+  namespace: ${NAMESPACE}
 spec:
   secretName: n8n-tls
   dnsNames:
-    - n8n.sttlab.local
+    - ${N8N_HOSTNAME}
   issuerRef:
-    name: sttlab-local-ca
-    kind: ClusterIssuer
+    name: ${TLS_ISSUER_NAME}
+    kind: ${TLS_ISSUER_KIND}