Keep large sidebars responsive (#1966)

* perf(app): keep large sidebars responsive

Sidebar rows rescanned every agent for each session update, multiplying work with both workspace and agent count. Build workspace activity once and share the derived sidebar model so updates scale with the underlying collections.

* test(app): use valid agent fixture for workspace restore

* refactor(app): keep sidebar model surface focused

packages/app/src/utils/navigate-to-agent/restore-archived-workspace.test.ts

@@ -27,6 +27,39 @@ const SERVER_ID = "server-1";
 const AGENT_ID = "agent-1";
 const WORKSPACE_ID = "workspace-1";
 
+function agent(archivedAt: Date | null): Agent {
+  const createdAt = new Date("2026-01-01T00:00:00.000Z");
+  return {
+    serverId: SERVER_ID,
+    id: AGENT_ID,
+    provider: "codex",
+    status: "idle",
+    createdAt,
+    updatedAt: createdAt,
+    lastUserMessageAt: null,
+    lastActivityAt: createdAt,
+    capabilities: {
+      supportsStreaming: true,
+      supportsSessionPersistence: true,
+      supportsDynamicModes: true,
+      supportsMcpServers: true,
+      supportsReasoningStream: true,
+      supportsToolInvocations: true,
+    },
+    currentModeId: null,
+    availableModes: [],
+    pendingPermissions: [],
+    persistence: null,
+    title: null,
+    cwd: "/repo",
+    workspaceId: WORKSPACE_ID,
+    model: null,
+    archivedAt,
+    parentAgentId: null,
+    labels: {},
+  };
+}
+
 function status(): "restoring" | "failed" | "needs-host-upgrade" | null {
   return (
     useSessionStore.getState().sessions[SERVER_ID]?.restoringWorkspaces.get(WORKSPACE_ID) ?? null
@@ -44,11 +77,7 @@ function seedArchivedAgent(options?: { worktreeRestore?: boolean }): void {
   } as unknown as Parameters<typeof store.updateSessionServerInfo>[1]);
   store.setAgents(SERVER_ID, (prev) => {
     const next = new Map(prev);
-    next.set(AGENT_ID, {
-      id: AGENT_ID,
-      workspaceId: WORKSPACE_ID,
-      archivedAt: new Date(),
-    } as unknown as Agent);
+    next.set(AGENT_ID, agent(new Date("2026-01-02T00:00:00.000Z")));
     return next;
   });
 }
@@ -95,11 +124,7 @@ describe("restoreArchivedWorkspace via navigateToAgent", () => {
     const store = useSessionStore.getState();
     store.setAgents(SERVER_ID, (prev) => {
       const next = new Map(prev);
-      next.set(AGENT_ID, {
-        id: AGENT_ID,
-        workspaceId: WORKSPACE_ID,
-        archivedAt: undefined,
-      } as unknown as Agent);
+      next.set(AGENT_ID, agent(null));
       return next;
     });
     refreshAgent.mockImplementation(() => new Promise(() => {}));