fix(app): subagents track display + archive flicker

- Bucket finished subagents as `done` instead of `attention`; drop
  "needs attention" segment from the section header — subagents are
  transient and don't carry an unread state worth surfacing.
- Gate the subagents selector on the existing pending-archive query so
  optimistic archives survive authoritative agent snapshots that wipe
  the local `archivedAt`. Last subagent archived now cleanly hides the
  whole track via the existing empty-rows guard.

packages/app/src/stores/workspace-subagents-integration.test.ts

@@ -209,10 +209,14 @@ describe("workspace subagents integration", () => {
 
     expect(getWorkspaceTabIds(workspaceKey!)).toEqual(["agent_parent-agent"]);
     expect(
-      selectSubagentsForParent(useSessionStore.getState(), {
-        serverId: SERVER_ID,
-        parentAgentId: "parent-agent",
-      }).map((row) => row.id),
+      selectSubagentsForParent(
+        useSessionStore.getState(),
+        {
+          serverId: SERVER_ID,
+          parentAgentId: "parent-agent",
+        },
+        new Set(),
+      ).map((row) => row.id),
     ).toEqual(["child-agent"]);
   });
 });