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/deployment.yaml

@@ -0,0 +1,21 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: myapp
+  labels:
+    app: myapp
+    managed-by: argocd
+spec:
+  selector:
+    matchLabels:
+      app: myapp
+  template:
+    metadata:
+      labels:
+        app: myapp
+    spec:
+      containers:
+      - name: myapp
+        image: nginx:alpine
+        ports:
+        - containerPort: 80