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/server/src/server/session.ts

@@ -514,7 +514,11 @@ export class Session {
           timestamp: new Date().toISOString(),
         } as const;
 
-        console.log(`[SERVER AGENT_STREAM] timestamp=${Date.now()} agentId=${event.agentId} eventType=${event.event.type}`);
+        const itemType = event.event.type === "timeline" ? event.event.item.type : undefined;
+        const logMessage = itemType
+          ? `[SERVER AGENT_STREAM] timestamp=${Date.now()} agentId=${event.agentId} eventType=${event.event.type} itemType=${itemType}`
+          : `[SERVER AGENT_STREAM] timestamp=${Date.now()} agentId=${event.agentId} eventType=${event.event.type}`;
+        console.log(logMessage);
         this.emit({
           type: "agent_stream",
           payload,