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/tool-subagent-report/tsconfig.json

@@ -17,6 +17,9 @@
     {
       "path": "../../llm/llm"
     },
+    {
+      "path": "../../core/system-prompt"
+    },
     {
       "path": "../../core/tools"
     },