Why: child runs had no independent transcript, no resume path (sealed
interrupted only), in-memory completion notices lost on restart, and
the subagent Background declaration had no consumer.
Evidence: go test -count=1 ./... green; TestSubagentTranscriptFilesWritten
(request/transcript/done files), TestResumeInterruptedChildrenRespawns
(re-spawn with original task + transcript-path hint, resumed.json
idempotent), TestNoticesPersistAndRedeliverAfterRestart (persist →
reload → deliver → delete), migration v17 pending_notices with up/down
on both dialects.
Impact: per-run transcript dirs under the workspace root (auxiliary,
write failures never abort spawns); interrupted children with a stored
request re-spawn as background runs at startup (pre-WP5 runs stay
sealed); notices survive restart; delegate factory honors
Request.Background with a sync fallback when the host lacks background
support.
Refs: Agent-Native#29, Agent-Squad#33, N2 plan WP5