fix(acp): await child background work

[danielkov]

Summary

Makes nested Kit ACP turns wait for child-owned background work and synthesize its results before resolving subagent, prompt, or fork. Applies structured completion to ACP v1 and v2 while preserving depth-0 explicit background behavior.

Motivation

A child could emit a provisional final response while a detached descendant was still running, causing the root-visible call to finish and leaving the descendant unsupervised. Fixes #28.

Impact

Nested subagent calls remain active until their background work reaches quiescence. Cancelling or closing the owning session now cooperatively cancels descendants and force-cancels tasks that do not settle within the bounded grace period.

Technical details

Structured completion boundary

Only ACP drivers started below depth 0 use structured completion. At a provisional terminal boundary, the actor waits for owned background work, consumes the published result, and drives a replacement synthesis turn until the child returns a final response with no active work.

Race-free completion handoff

Background jobs track activity generations and terminal publication by tool call ID. The actor cannot resolve between execution finishing and the task manager publishing the corresponding completion event, avoiding invisible or duplicate autonomous continuations.

Validation

• cargo fmt --all -- --check
• cargo test --quiet
• git diff --check