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/stores/session-store.ts

@@ -37,6 +37,10 @@ import {
   createAgentLastActivityCoalescer,
   type AgentLastActivityCommitter,
 } from "@/runtime/activity";
+import {
+  buildWorkspaceAgentActivityIndex,
+  type WorkspaceAgentActivity,
+} from "@/utils/workspace-agent-activity";
 
 // Re-export types that were in session-context
 export type MessageEntry =
@@ -353,6 +357,7 @@ export interface SessionState {
 
   // Agents
   agents: Map<string, Agent>;
+  workspaceAgentActivity: Map<string, WorkspaceAgentActivity>;
   agentDetails: Map<string, Agent>;
   workspaces: Map<string, WorkspaceDescriptor>;
   // Project parents with no active workspaces, keyed by projectId. The
@@ -560,6 +565,7 @@ function createInitialSessionState(serverId: string, client: DaemonClient): Sess
     agentAuthoritativeHistoryApplied: new Map(),
     initializingAgents: new Map(),
     agents: new Map(),
+    workspaceAgentActivity: new Map(),
     agentDetails: new Map(),
     workspaces: new Map(),
     emptyProjects: new Map(),
@@ -1194,7 +1200,11 @@ export const useSessionStore = create<SessionStore>()(
             ...prev,
             sessions: {
               ...prev.sessions,
-              [serverId]: { ...session, agents: nextAgents },
+              [serverId]: {
+                ...session,
+                agents: nextAgents,
+                workspaceAgentActivity: buildWorkspaceAgentActivityIndex(nextAgents),
+              },
             },
           };
         });