feat: let a workflow act as its agent

The agents' Keycloak client secrets reach the pod through the keycloak store,
naming the one property each workflow needs rather than copying it into n8n's
own Vault path — one source for a client secret, so rotating it in Vault
rotates it everywhere.

N8N_BLOCK_ENV_ACCESS_IN_NODE=false so a workflow can read them as $env, which
keeps them out of the committed workflow JSON. The trade-off is written down in
the ConfigMap: any workflow can then read any variable in the pod, which is
acceptable while this instance runs only this estate's agents.

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

k8s/n8n/deployment.yaml

@@ -72,6 +72,11 @@ spec:
           envFrom:
             - configMapRef:
                 name: n8n-config
+            # The agents' Keycloak client secrets, reached from a workflow as
+            # `$env.APPROVISIONNEMENT_CLIENT_SECRET`. A workflow acts as its
+            # agent, so it is the workflow that must be able to exchange tokens.
+            - secretRef:
+                name: n8n-agent-secrets
           volumeMounts:
             - name: data
               mountPath: /home/node/.n8n