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/agent/README.md

@@ -58,6 +58,8 @@ Inbox live notifications are deliberately per-message and minimal: `agent/inbox/
 
 Turn and step boundaries and the model token stream are durable `session/event` facts rather than mirrored `agent/*` notifications. Consumers read `turn/*`, `step/*`, and `assistant/chunk` from the session feed; tool policy and outcome observation belong to the complete pipeline documented by [`dsh-tools`](../tools/README.md).
 
+`foldConsumedWork(events)` reads that feed back for the one question the turn sequence cannot answer alone: what became of the work a log consumed. It returns the latest `turn/end` that accounts for consumed work — a turn that entered a model step, or one that claimed inbox input and then failed, was stopped, or was rejected before reaching one — plus whether accepted work was later cancelled out of the inbox unrun. Both facts come from the log, so a cancellation reads the same whichever owner issued it. A no-step turn that took nothing, or emptied its claim and completed, describes no work and is skipped; a `blocked` end over claimed input is an account, because rejection discarded that input.
+
 ### Agent interface (`types.ts`)
 
 The handle every plugin programs against: