fix(background-task): return result instead of status for completed tasks

- Fix background_output to check completion status before block flag
- Update call_omo_agent return message to correctly indicate block=false as default
- Add system notification guidance in return message

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

src/tools/call-omo-agent/tools.ts

@@ -67,9 +67,10 @@ Description: ${task.description}
 Agent: ${task.agent} (subagent)
 Status: ${task.status}
 
-Use \`background_output\` tool with task_id="${task.id}" to check progress or retrieve results.
-- block=false: Check status without waiting
-- block=true (default): Wait for completion and get result`
+The system will notify you when the task completes.
+Use \`background_output\` tool with task_id="${task.id}" to check progress:
+- block=false (default): Check status immediately - returns full status info
+- block=true: Wait for completion (rarely needed since system notifies)`
   } catch (error) {
     const message = error instanceof Error ? error.message : String(error)
     return `Failed to launch background agent task: ${message}`