fix(task): return task_id when subagent run errors

[weiguangli-io]

Fixes #13910

Summary

This keeps the task tool recoverable when a subagent run fails.

• wrap SessionPrompt.prompt() in packages/opencode/src/tool/task.ts with error handling
• on error, return structured output that still includes task_id
• keep success behavior unchanged
• add focused tests for both success and failure paths

Why

Issue #13910 reports that when subagent execution throws (cancel/timeout/provider error), the parent agent loses the session handle and cannot resume with task_id.

This change guarantees that task_id is always returned, so the parent can retry/inspect the same subagent session.

Fixes #13910

Testing

• cd packages/opencode && bun test test/tool/task.test.ts
• cd packages/opencode && bun run typecheck

(Repository-wide pre-push hook typecheck fails in this local environment due missing @typescript/native-preview-darwin-x64, but the package-local checks above passed.)

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[weiguangli-io]

Closing due to inactivity. Happy to reopen if there's interest.

[ELLIOTTCABLE]

There's interest from me, at least; I don't currently see any way to correct an in-flight subagent doing something wrong, without losing all of its context and work?

All three of the approaches I am used to for this in Claude Code seem to be absent:

• I cannot send a message to a subagent anywhere in the TUI that I could find;
• the conductor tells me it has no 'message the running subagent' tool to call;
• and interrupting (which I seem to only be able to do to the whole tree at once, not just the subagent, which feels like another serious oversight) doesn't produce a task_id allowing the parent to resume it with the steering info.

[tamasys]

@weiguangli-io Interest from me too! Seems crazy that this hasn't been implemented before, it's so frustrating having to choose between losing all subagent context or allowing it to keep running on a misguided path.