Document app-wide system prompt

Author: bavadim

README.md

@@ -61,7 +61,9 @@ const adapter = createOpenAIResponsesAdapter({
 // Adapter shape:
 // { model, generate, countTokens?, contextWindowTokens? }
 
-const agentMessages = createAgentMessages();
+const agentMessages = createAgentMessages(
+  "System: This demo is for creating beautiful interfaces. Focus on elegant layout, typography, and clear visual hierarchy."
+);
 const tools = [
   jsInterpreterTool(),
 ];