fix: remove secret.yaml from kustomization files (gitignored)

ArgoCD kustomize build failed because secret.yaml was referenced in
kustomization.yaml but excluded via .gitignore. Secrets are managed
directly in-cluster and must not be part of the kustomize build.

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

k8s/omada-mcp/kustomization.yaml

@@ -6,6 +6,5 @@ namespace: omada-mcp
 
 resources:
   - namespace.yaml
-  - secret.yaml
   - deployment.yaml
   - service.yaml