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

examples/acp-agent/tests/snapshots/subagent-depth-two-rejection/input.json

@@ -0,0 +1,7 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSession" },
+    { "op": "prompt", "text": "Delegate through two child generations. The depth-two child must attempt one more subagent call and report the rejection." }
+  ]
+}