The native subagent tool creates durable child chats and can run in a parallel tool batch, but waits for a final answer. The reviewed work also needed partial findings, follow-up instructions, selective interruption, and continued parent work. Opening child chats in the UI (#19) does not supply those operations to the parent agent.

## Acceptance

- Start child work while the parent continues; list/status children and inspect partial findings. - Send corrections and follow-up tasks to an existing child; wait for and collect its eventual result. - Stop selected work and preserve parent stop/kill propagation and durable ownership across reconnect/restart. - Read-only reviews may share a lane; writing chats use separate lanes with one writer per lane. - Replay a real parallel review: redirect one child, stop another, inspect the remaining child, and integrate findings entirely in Ace.

Use pi's existing child chats/tasks rather than another agent state store. Preserve the current one-shot tool where useful. Recursive child creation is separate unless the selected workflow needs it.

Dogfooding core/QOL: #5. Related: #19, #24. [Review evidence](https://github.com/githubnext/ace2/issues/5#issuecomment-5986485157).