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.

examples/headless-agent/cordis.yml

@@ -118,12 +118,16 @@
     backgroundMode: continuable
     maxDepth: 1
 
+# Fork stays one-shot because a continuable child's `report` tool and prompt
+# section precede the inherited history a fork reuses; `run_in_background` is off
+# because this example mounts no task service. See .agents/notes/implemented/architecture/2026-08-10-fork-children-stay-one-shot.md.
 - id: tool-subagent-fork
   name: '@deepseek-ai/dsh-tool-subagent'
   config:
     provider: fork
     toolName: subagent_fork
-    backgroundMode: continuable
+    backgroundMode: one-shot
+    enableRunInBackground: false
     maxDepth: 1
 
 # The worker-thread workflow engine fans a model-written JavaScript script's