feat: restructure GitOps for multi-environment deployment

- Add apps/myapp/base with deployment, service, httproute
- Add apps/myapp/overlays/staging (1 replica, 128Mi/256Mi, staging hostname)
- Add apps/myapp/overlays/production (2 replicas, 256Mi/512Mi, prod hostname)
- Remove old apps/sample-webapp structure

Structure follows folder-per-environment pattern with Kustomize overlays.

apps/myapp/base/kustomization.yaml

@@ -0,0 +1,7 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+- deployment.yaml
+- service.yaml
+- httproute.yaml