The spawned-agent task now pushes an agent_completed/agent_failed event
into the spawning context's notification queue before returning, so a
parent that keeps working learns mid-turn that a child finished instead
of discovering it only at the turn-end guardrail. Cancelled or
already-collected agents are suppressed by the existing drain-time
registration filter. This delivery applies regardless of whether
background jobs are enabled.
- add NotificationQueue/SystemNotification: every context owns a fresh
queue (children never inherit the parent's, avoiding first-drainer-wins
races between transcripts)
- job tasks push job_completed/job_failed events on completion, failure,
and timeout; a panic skips the push and is surfaced by the guardrail's
finished-handle enumeration and collect's JoinError mapping instead
- events for jobs already collected or cancelled are dropped at drain time
by filtering against live supervisor registration
- replace inject_escalation_notification with single-pass
merge_system_channel: pending_escalations (root-only) ordered before
system_notifications (any depth) on the last tool result of a batch;
byte-identical output when notifications are empty, proven by the
unmodified pre-merger characterization tests
Dark-Alex-17