fix(session-notification): skip notification for subagent sessions (#70)

- Import subagentSessions from claude-code-session-state in both manager.ts and session-notification.ts
- Add sessionID to subagentSessions Set when creating background task session
- Remove sessionID from subagentSessions when background task session is deleted
- Check if session is in subagentSessions before triggering notification

Fixes #70: Notification hook no longer triggers for subagent idle events

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/hooks/session-notification.ts

@@ -1,5 +1,6 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { platform } from "os"
+import { subagentSessions } from "../features/claude-code-session-state"
 
 interface Todo {
   content: string
@@ -219,6 +220,8 @@ export function createSessionNotification(
       const sessionID = props?.sessionID as string | undefined
       if (!sessionID) return
 
+      if (subagentSessions.has(sessionID)) return
+
       if (notifiedSessions.has(sessionID)) return
       if (pendingTimers.has(sessionID)) return