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/kustomization.yaml

@@ -2,10 +2,8 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
   - namespace.yaml
-  - ca-configmap.yaml
   - secrets/clustersecretstore.yaml
   - secrets/externalsecret.yaml
-  - secrets/agent-externalsecret.yaml
   - postgres/statefulset.yaml
   - postgres/service.yaml
   - n8n/pvc.yaml