fix(karrot): fix nginx DNS resolver for Kubernetes and add README

The karrot-frontend nginx template hardcodes Docker's DNS resolver
(127.0.0.11) which doesn't exist in Kubernetes. Added a ConfigMap to
override the template, removing the Docker DNS resolver by using a
direct proxy_pass with no nginx variable (which would force runtime
resolution). Also adds a README with usage instructions.

Documents lessons learned in ADDING-APPS-NOTES.md:
- Note 24: nginx Docker DNS resolver doesn't work in Kubernetes; any
  nginx variable in proxy_pass (including $request_uri) forces runtime
  DNS resolution requiring a resolver directive
- Note 25: ConfigMap changes don't restart pods; subPath mounts never
  auto-update; always follow with kubectl rollout restart
- Note 26: includeSelectors mismatch affects every deployment in an
  app — check all endpoints, not just the web-facing one

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

karrot/versions/1/kustomization.yaml

@@ -9,6 +9,7 @@ labels:
       partOf: wild-cloud
 resources:
   - namespace.yaml
+  - nginx-template-configmap.yaml
   - db-init-job.yaml
   - deployment-backend.yaml
   - deployment-frontend.yaml