perf: prevent HomeScreen rerenders during agent streaming

- Add top-level agentLastActivity Map to store (prevents cascade rerenders)
- Remove lastActivityAt updates from agent_stream handler (only status changes update timestamps)
- useAggregatedAgents now uses agent.lastActivityAt directly
- Remove investigation logging from previous debug session

The root cause was that every stream event updated agent.lastActivityAt,
causing useAggregatedAgents to recalculate, which caused HomeScreen to
rerender even when not visible. Now timestamps only update on meaningful
events (agent status changes), not individual stream tokens.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/app/src/stores/session-store.ts

@@ -243,6 +243,9 @@ interface SessionStoreState {
 
   // Single draft for the create agent modal
   createModalDraft: DraftInput | null;
+
+  // Agent activity timestamps (top-level, keyed by agentId to prevent cascade rerenders)
+  agentLastActivity: Map<string, Date>;
 }
 
 // Action types
@@ -272,6 +275,9 @@ interface SessionStoreActions {
   // Agents
   setAgents: (serverId: string, agents: Map<string, Agent> | ((prev: Map<string, Agent>) => Map<string, Agent>)) => void;
 
+  // Agent activity timestamps
+  setAgentLastActivity: (agentId: string, timestamp: Date) => void;
+
   // Commands
   setCommands: (serverId: string, commands: Map<string, Command> | ((prev: Map<string, Command>) => Map<string, Command>)) => void;
 
@@ -363,6 +369,7 @@ export const useSessionStore = create<SessionStore>()(
     sessions: {},
     drafts: new Map(),
     createModalDraft: null,
+    agentLastActivity: new Map(),
 
     // Session management
     initializeSession: (serverId, ws, audioPlayer) => {
@@ -573,6 +580,22 @@ export const useSessionStore = create<SessionStore>()(
       });
     },
 
+    // Agent activity timestamps (top-level, does NOT mutate session object)
+    setAgentLastActivity: (agentId, timestamp) => {
+      set((prev) => {
+        const currentTimestamp = prev.agentLastActivity.get(agentId);
+        if (currentTimestamp && currentTimestamp.getTime() === timestamp.getTime()) {
+          return prev;
+        }
+        const nextActivity = new Map(prev.agentLastActivity);
+        nextActivity.set(agentId, timestamp);
+        return {
+          ...prev,
+          agentLastActivity: nextActivity,
+        };
+      });
+    },
+
     // Commands
     setCommands: (serverId, commands) => {
       set((prev) => {
@@ -775,19 +798,22 @@ export const useSessionStore = create<SessionStore>()(
 
     // Agent directory - derived from agents (computed on-demand)
     getAgentDirectory: (serverId) => {
-      const session = get().sessions[serverId];
+      const state = get();
+      const session = state.sessions[serverId];
       if (!session) {
         return undefined;
       }
 
       const entries: AgentDirectoryEntry[] = [];
       for (const agent of session.agents.values()) {
+        // Get lastActivityAt from top-level slice, fallback to agent.lastActivityAt
+        const lastActivityAt = state.agentLastActivity.get(agent.id) ?? agent.lastActivityAt;
         entries.push({
           id: agent.id,
           serverId,
           title: agent.title ?? null,
           status: agent.status,
-          lastActivityAt: agent.lastActivityAt,
+          lastActivityAt,
           cwd: agent.cwd,
           provider: agent.provider,
         });