fix(tool-subagent): enforce depth only at runtime

examples/acp-agent/depth-two.cordis.yml

@@ -0,0 +1,13 @@
+# Depth-limit snapshot overlay: keep the default composition and allow two
+# generations of spawn children before runtime enforcement rejects another.
+- id: base
+  name: '@cordisjs/plugin-include'
+  config:
+    path: ./cordis.yml
+    patches:
+      - id: tool-subagent
+        name: '@deepseek-ai/dsh-tool-subagent'
+        config:
+          provider: spawn
+          toolName: subagent
+          maxDepth: 2