fix(task): return task_id when subagent run errors - #13958

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

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

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.

@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.

@weiguangli-io showing interest here.

i see no reason why this feature shouldn't be implemented, and a lot of reasons why it should. i mean, why should sub-agents lose all of their work on session interruptions? and what's the reasoning behind keeping this behavior?

to me it seems like a purely QoL feature that would prevent having to re-start interrupted sub-agents when they could instead be resumed.