feat(delegate): Non-blocking background subagent execution

Summary

_delegate_tasks() in openhands-tools/openhands/tools/delegate/impl.py uses thread.join() to block the parent agent until the sub-agent completes. This prevents the parent from doing other work while waiting for long-running sub-agent tasks.

Proposed Solution

Add a background flag to delegation that returns immediately with task IDs, plus commands for polling status, retrieving output, and stopping background agents.

Key Requirements

• Add background: bool parameter to delegation
• When background=True, return immediately with a task ID
• Implement check_status(task_id) to poll sub-agent state
• Implement get_output(task_id) to retrieve results when complete
• Implement stop(task_id) to terminate a running background agent
• Track background agents in a registry for the parent context

Key Files

• openhands-tools/openhands/tools/delegate/impl.py — _delegate_tasks() with thread.join()

Acceptance Criteria

• background flag is supported in delegation
• Parent agent is not blocked when background=True
• Task ID is returned for background agents
• Status polling, output retrieval, and stop commands work
• Multiple background agents can run concurrently

Start in openhands-tools/openhands/tools/delegate/impl.py at _delegate_tasks() and its thread.join() behavior. Trace how delegation state and parent context are represented, then implement the background task lifecycle so task IDs, status, output retrieval, stopping, and concurrent agents satisfy every acceptance criterion.