Archive finished subagents across the whole track (#3368)

* feat(app): archive finished subagents together

Make the track action clean up finished managed and provider-owned children as one operation while preserving running work. Managed archives remain sequential and provider-owned dismissal stays client-local.

* test(app): clarify subagent archive assertions

packages/app/src/subagents/select.test.ts

@@ -116,7 +116,7 @@ describe("selectSubagentsForParent", () => {
         toolCallId: "call-1",
       },
     });
-    store.hideFinishedForParent(SERVER_ID, "parent-a");
+    store.hideFromTrack(SERVER_ID, "parent-a", ["provider-child"]);
 
     expect(
       selectProviderSubagentsForParent(