[WS5] Contribute async task refactor to upstream (PR #7206 / issue #5887)

[randomm]

Parent: #158

Objective

After completing WS1 (refactoring our async task tool to minimal-invasion pattern), contribute the result back to upstream OpenCode via PR anomalyco#7206 or a new PR against issue anomalyco#5887.

Why Contribute

• Upstream issue [feat] True Async/Background Sub-Agent Delegation anomalyco/opencode#5887 has community support for async sub-agent delegation
• PR feat(tui): fire-and-forget async subagent tasks anomalyco/opencode#7206 exists but is 2,046 commits behind dev with no maintainer review
• If upstream merges async tasks, our fork maintenance burden drops to near zero for the hardest component
• Estimated upstream timeline without our help: 2-4 months

Prerequisites

• WS1 complete — our refactored implementation matches PR feat(tui): fire-and-forget async subagent tasks anomalyco/opencode#7206's minimal pattern
• Clean, well-documented code with tests
• Backward compatibility (blocking mode as default, async as opt-in)
• No fork-specific dependencies

Steps

1. Review PR feat(tui): fire-and-forget async subagent tasks anomalyco/opencode#7206 current state and maintainer feedback
2. Rebase our refactored implementation onto upstream dev
3. Either:
   • Comment on PR feat(tui): fire-and-forget async subagent tasks anomalyco/opencode#7206 offering to co-author/rebase
   • Open new PR if feat(tui): fire-and-forget async subagent tasks anomalyco/opencode#7206 is abandoned
4. Engage with upstream maintainers on design review
5. Iterate based on feedback

Acceptance Criteria

• PR submitted to upstream
• Addresses any review feedback
• Merged upstream (stretch goal)