feat(hook): add session-notification hook for agent idle state notifications

- Implement createSessionNotification() for macOS/Linux/Windows notifications
- Debounce timer to confirm session still idle after 1.5s (prevents false alerts from todo-continuation)
- Skip notification if incomplete todos exist (smart coordination with todo-continuation-enforcer)
- Support cross-platform alerts (osascript, notify-send, PowerShell)
- Configurable title, message, sound, and idle confirmation delay

src/hooks/index.ts

@@ -1,2 +1,3 @@
 export * from "./todo-continuation-enforcer"
 export * from "./context-window-monitor"
+export * from "./session-notification"