feat(tool-subagent): default maxDepth 1 with at-cap schema hiding

An omitted maxDepth meant unbounded recursion, and the shipped examples
shipped that default. maxDepth now defaults to 1; a numeric cap requires
the provider's depthLimit capability (the mount fails loud and points to
the explicit 'provider-managed' opt-out for out-of-process providers),
and a child AT the cap loses the delegating tool from its own schema via
the child toolFilter — prompt-face hiding on top of the execution-face
depth check. Examples pin maxDepth explicitly. The ACP snapshot harness
gains Scenario.childToolOmissions so a child session may legitimately
omit declared delegation tools from its pinned header and prompt;
affected subagent/workflow goldens are re-recorded.

examples/headless-agent/cordis.yml

@@ -64,12 +64,14 @@
   config:
     provider: spawn
     toolName: subagent
+    maxDepth: 1
 
 - id: tool-subagent-fork
   name: '@deepseek-ai/dsh-tool-subagent'
   config:
     provider: fork
     toolName: subagent_fork
+    maxDepth: 1
 
 # The worker-thread workflow engine fans a model-written JavaScript script's
 # `agent()` calls out through the spawn backend.