multitenant-down

Tear down the multi-tenant K8s stack: 1. Kill all port-forwards: ```bash pkill -f 'port-forward.*duckgres' 2>/dev/null ``` 2. Delete the duckgres namespace (removes control plane, workers, RBAC, services): ```bash kubectl delete namespace duckgres --ignore-not-found ``` 3. Stop the config store: ```bash docker compose -f k8s/local-config-store.compose.yaml down ``` 4. Clean up the TLS cert: ```bash rm -f /tmp/duckgres-server.crt ``` 5. Report to the user that the stack is torn down.