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/workspace-agent-activity.ts

@@ -0,0 +1,37 @@
+import type { Agent, WorkspaceDescriptor } from "@/stores/session-store";
+import { deriveSidebarStateBucket } from "./sidebar-agent-state";
+
+export interface WorkspaceAgentActivity {
+  status: WorkspaceDescriptor["status"];
+  enteredAt: Date | null;
+}
+
+export function buildWorkspaceAgentActivityIndex(
+  agents: ReadonlyMap<string, Agent>,
+): Map<string, WorkspaceAgentActivity> {
+  const activityByWorkspaceId = new Map<string, WorkspaceAgentActivity>();
+
+  for (const agent of agents.values()) {
+    if (agent.archivedAt || agent.parentAgentId || !agent.workspaceId) {
+      continue;
+    }
+
+    const enteredAt = agent.attentionTimestamp ?? agent.updatedAt;
+    const current = activityByWorkspaceId.get(agent.workspaceId);
+    if (current && enteredAt <= (current.enteredAt ?? new Date(0))) {
+      continue;
+    }
+
+    activityByWorkspaceId.set(agent.workspaceId, {
+      status: deriveSidebarStateBucket({
+        status: agent.status,
+        pendingPermissionCount: agent.pendingPermissions.length,
+        requiresAttention: agent.requiresAttention,
+        attentionReason: agent.attentionReason,
+      }),
+      enteredAt,
+    });
+  }
+
+  return activityByWorkspaceId;
+}