fix(delegate-task): report the real background task id

Keep background task metadata aligned with the background_output contract so callers do not pass a session id where the task manager expects a background task id.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/delegate-task/background-task.ts

@@ -82,7 +82,7 @@ export async function executeBackgroundTask(
     }
 
     const taskMetadataBlock = sessionId
-      ? `\n\n<task_metadata>\nsession_id: ${sessionId}\ntask_id: ${sessionId}\nbackground_task_id: ${task.id}\n</task_metadata>`
+      ? `\n\n<task_metadata>\nsession_id: ${sessionId}\ntask_id: ${task.id}\nbackground_task_id: ${task.id}\n</task_metadata>`
       : ""
 
     return `Background task launched.