refactor(background-notification): remove chat.message handler

- Remove unused chat message notification handler
- Remove formatDuration and formatNotifications helpers
- Simplify to event-only handling
- Remove chat.message hook call from main plugin

Background task notifications now rely on event-based system only.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/index.ts

@@ -180,7 +180,6 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
 
     "chat.message": async (input, output) => {
       await claudeCodeHooks["chat.message"]?.(input, output);
-      await backgroundNotificationHook["chat.message"](input, output);
     },
 
     config: async (config) => {