feat(subagent): deliver continuable child settlement to parents

A continuable child that stopped without reporting — an error, a token
ceiling, cancellation, teardown — left its parent nothing to act on.
The continuation manager now delivers an unconditional settlement
notice to the durable direct parent before releasing ownership, folding
consumed work (foldConsumedWork supersedes findLastMessageTurnEnd) so a
claimed-but-unrun prompt reads as aborted rather than completed, waking
an idle parent, steering a busy one, and never waking a closing tree.

packages/core/session/src/index.ts

@@ -34,27 +34,6 @@ export { deriveEventMessage, foldSurface, isAppendSurfaceEvent, isReplacementSur
 export { canonicalHeader, foldRequestHeader, headerEquals } from './request-header.ts'
 export { KNOWN_SESSION_EVENT_TYPES } from './known-event-types.ts'
 
-/**
- * Find the latest closed turn that entered at least one model step, ignoring
- * balanced no-step turns produced by rejection, empty input, or cancellation.
- * @param events - session events, or an owned suffix, to inspect.
- * @returns the latest matching turn end, or `undefined`.
- */
-export function findLastMessageTurnEnd(
-  events: readonly SessionEvent[],
-): SessionEvent<'turn/end'> | undefined {
-  const steppedTurns = new Set<number>()
-  let latest: SessionEvent<'turn/end'> | undefined
-  for (const event of events) {
-    if (event.type === 'step/start') {
-      steppedTurns.add(event.data.turn)
-      continue
-    }
-    if (event.type === 'turn/end' && steppedTurns.delete(event.data.turn)) latest = event
-  }
-  return latest
-}
-
 declare module '@deepseek-ai/cordis' {
   interface Context {
     sessions: SessionStore