fix(background-task): change default block to false and clarify system notification

- Change background_output default from block=true to block=false
- Add documentation about system auto-notification on task completion
- Clarify that block=false returns full status info, not empty

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

src/tools/background-task/constants.ts

@@ -18,16 +18,19 @@ export const BACKGROUND_OUTPUT_DESCRIPTION = `Get output from a background task.
 
 Arguments:
 - task_id: Required task ID to get output from
-- block: If true (default), wait for task completion. If false, return current status immediately.
+- block: If true, wait for task completion. If false (default), return current status immediately.
 - timeout: Max wait time in ms when blocking (default: 60000, max: 600000)
 
 Returns:
+- When not blocking: Returns current status with task ID, description, agent, status, duration, and progress info
 - When blocking: Waits for completion, then returns full result
-- When not blocking: Returns current status and progress
+
+IMPORTANT: The system automatically notifies the main session when background tasks complete.
+You typically don't need block=true - just use block=false to check status, and the system will notify you when done.
 
 Use this to:
-- Check task progress (block=false)
-- Wait for and retrieve task result (block=true)
+- Check task progress (block=false) - returns full status info, NOT empty
+- Wait for and retrieve task result (block=true) - only when you explicitly need to wait
 - Set custom timeout for long tasks`
 
 export const BACKGROUND_CANCEL_DESCRIPTION = `Cancel a running background task.