feat(subagent): make final reports the continuable return contract

A continuable child could finish without giving its parent a usable
result: the report tool was described as optional and its default
delivery stayed quiet. Install the child-scoped report obligation (tool
description plus the tool:report prompt section), default reportDelivery
to wakeup, bind shipped subagent_fork delegations to one-shot so fork
keeps its prefix reuse, and extend the ACP snapshot harness to pin child
system prompts alongside child tool schemas.

packages/subagent/subagent-fork/src/index.ts

@@ -74,6 +74,12 @@ class ForkProvider implements SubagentProvider {
     })
   }
 
+  // TODO(fork-continuable-prefix-reuse): no shipped composition calls this —
+  // they bind fork to `backgroundMode: one-shot` because a continuable child's
+  // `report` tool and prompt section precede the inherited history, defeating
+  // the prefix reuse a fork exists for. Reopening needs a byte-identical child
+  // system prompt and tool schemas; see issue #2124 and
+  // .agents/notes/implemented/architecture/2026-08-10-fork-children-stay-one-shot.md.
   prepareContinuable(request: ContinuableCreateRequest): Promise<ContinuableCreateSpec> {
     // The fork prefix is captured ONCE, at creation: it becomes part of the
     // child's own durable transcript, so a later cold resume replays that