Add app label to Deployment metadata for label-based cleanup

Deployments were the only resource type missing the `app` label on
their metadata. Services, ConfigMaps, PVCs, and Jobs all had it.
Without it, _delete_resources_by_label() in down() couldn't find
Deployments when cleaning up in a shared namespace.

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

stack_orchestrator/deploy/k8s/cluster_info.py

@@ -687,7 +687,10 @@ class ClusterInfo:
         deployment = client.V1Deployment(
             api_version="apps/v1",
             kind="Deployment",
-            metadata=client.V1ObjectMeta(name=f"{self.app_name}-deployment"),
+            metadata=client.V1ObjectMeta(
+                name=f"{self.app_name}-deployment",
+                labels={"app": self.app_name, **({"app.kubernetes.io/stack": self.stack_name} if self.stack_name else {})},
+            ),
             spec=spec,
         )
         return deployment