feat: include subagent in task_metadata when resuming sessions

When delegate-task resumes a session via session_id, the response
task_metadata now includes a subagent field identifying which agent
was running in the resumed session. This allows the parent agent to
know what type of subagent it is continuing.

- sync-continuation: uses resumeAgent extracted from session messages
- background-continuation: uses task.agent from BackgroundTask object
- Gracefully omits subagent when agent info is unavailable

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

@@ -50,7 +50,7 @@ Use \`background_output\` with task_id="${task.id}" to check progress.
 
 <task_metadata>
 session_id: ${task.sessionID}
-</task_metadata>`
+${task.agent ? `subagent: ${task.agent}\n` : ""}</task_metadata>`
   } catch (error) {
     return formatDetailedError(error, {
       operation: "Continue background task",