[Medium] Implement check_task tool for background task polling

[randomm]

Problem

Phase 1 of streaming subagents is incomplete. The check_task tool for polling background task status is not implemented.

Required Implementation

Tool that allows agents to:

1. Query status of a background task by ID
2. Get partial results if available
3. Check if task completed/failed

Files

• Create: packages/opencode/src/tool/check_task.ts
• Update: packages/opencode/src/tool/registry.ts

Reference

See TODO-streaming-subagents.md for full Phase 1 requirements.

Branch

claude/simplify-agentic-loop-2jRSP

[randomm]

Implemented in branch claude/simplify-agentic-loop-2jRSP. check_task tool exists at packages/opencode/src/tool/check_task.ts with status, duration, agent, and result fields.