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/tests/session.spec.ts

@@ -7,28 +7,11 @@ import SessionStore, {
   Session,
   SessionEvent,
   SessionId,
-  findLastMessageTurnEnd,
   snapshotSessionEvent,
 } from '@deepseek-ai/dsh-session'
 import type { CreateSessionOptions, SessionEventType, SessionHeader, SessionSurface, TodoItem } from '@deepseek-ai/dsh-session'
 
 describe('Session', () => {
-  it('finds the latest closed turn that entered a model step', () => {
-    const session = Session.create(SessionId('last-message-turn'))
-    session.append('turn/start', { turn: 1 })
-    session.append('turn/end', { turn: 1, reason: { kind: 'blocked' } })
-
-    expect(findLastMessageTurnEnd(session.events)).toBeUndefined()
-
-    session.append('turn/start', { turn: 2 })
-    session.append('step/start', { turn: 2, step: 1 })
-    session.append('step/end', { turn: 2, step: 1 })
-    session.append('turn/end', { turn: 2, reason: { kind: 'max-tokens' } })
-
-    expect(findLastMessageTurnEnd(session.events)?.data)
-      .toEqual({ turn: 2, reason: { kind: 'max-tokens' } })
-  })
-
   it('exposes one stable readonly surface view', () => {
     const session = Session.create(SessionId('surface-view'))
     const surface = session.surface