Add HelloWorld debug agent and per-chat handler

whatsapp-router/src/chatHandlers.ts

@@ -0,0 +1,15 @@
+import { helloWorldAgent } from './helloAgent';
+import { WhatsAppMessage } from './types';
+
+export type Handler = string | ((msg: WhatsAppMessage | string) => Promise<string>);
+
+export const chatHandlers: Record<string, Handler> = {
+  '50498554225@c.us': helloWorldAgent,
+  // Add other mappings like:
+  // '50496210031@c.us': 'http://llm-agent:8000'
+};
+
+export function getHandler(chatId: string | undefined, defaultUrl?: string): Handler | undefined {
+  if (!chatId) return defaultUrl;
+  return chatHandlers[chatId] || defaultUrl;
+}