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/configmap.yaml

@@ -4,21 +4,21 @@ apiVersion: v1
 kind: ConfigMap
 metadata:
   name: n8n-config
-  namespace: n8n
+  namespace: ${NAMESPACE}
 data:
-  GENERIC_TIMEZONE: Europe/Paris
-  TZ: Europe/Paris
+  GENERIC_TIMEZONE: ${TIMEZONE}
+  TZ: ${TIMEZONE}
   NODE_ENV: production
 
   # What n8n believes it is reachable at. It goes into the webhook URLs it
   # generates and into the links it puts in its own UI, so it must be the
   # ingress host and not the Service — a webhook advertised as `n8n` inside the
   # cluster is a webhook nothing outside can call.
-  N8N_HOST: n8n.sttlab.local
+  N8N_HOST: ${N8N_HOSTNAME}
   N8N_PORT: "5678"
   N8N_PROTOCOL: https
-  WEBHOOK_URL: https://n8n.sttlab.local/
-  N8N_EDITOR_BASE_URL: https://n8n.sttlab.local/
+  WEBHOOK_URL: https://${N8N_HOSTNAME}/
+  N8N_EDITOR_BASE_URL: https://${N8N_HOSTNAME}/
 
   # TLS terminates at the ingress, so n8n itself serves plain HTTP and must be
   # told it is behind a proxy — otherwise it reads the hop's address instead of
@@ -28,18 +28,14 @@ data:
   # The default is to keep every execution for ever, which turns the database
   # into a log. Two weeks is long enough to debug a failed run.
   EXECUTIONS_DATA_PRUNE: "true"
-  EXECUTIONS_DATA_MAX_AGE: "336"
+  EXECUTIONS_DATA_MAX_AGE: ${EXECUTIONS_MAX_AGE_HOURS}
 
   N8N_DIAGNOSTICS_ENABLED: "false"
   N8N_VERSION_NOTIFICATIONS_ENABLED: "false"
   N8N_RUNNERS_ENABLED: "true"
 
-  # Lets a workflow read `$env.<NAME>`. n8n blocks this by default, which is the
-  # right default for an instance where anyone can write a workflow — here it is
-  # how an agent's client secret reaches the workflow that acts as that agent,
-  # without the secret being stored in the workflow JSON and committed.
-  #
-  # It does mean any workflow can read any variable in the pod. That is
-  # acceptable while the instance runs this estate's own agents and nothing
-  # else; it stops being acceptable the moment it is shared.
-  N8N_BLOCK_ENV_ACCESS_IN_NODE: "false"
+  # "false" lets a workflow read `$env.<NAME>`, which is how a secret reaches a
+  # workflow without being stored in the workflow JSON. n8n blocks it by
+  # default, and that default is right wherever more than one team can write a
+  # workflow: any workflow can then read any variable in the pod.
+  N8N_BLOCK_ENV_ACCESS_IN_NODE: ${BLOCK_ENV_ACCESS}