chore: import upstream snapshot with attribution

tests/test_engine/test_spawn_depth_unified.py

@@ -0,0 +1,19 @@
+"""Spawn-depth constants must agree across the engine and gateway paths."""
+
+from __future__ import annotations
+
+from opensquilla.agents.limits import MAX_SPAWN_DEPTH
+from opensquilla.engine import subagent as engine_subagent
+from opensquilla.tools.builtin import sessions as sessions_tool
+
+
+def test_max_spawn_depth_is_three() -> None:
+    assert MAX_SPAWN_DEPTH == 3
+
+
+def test_engine_default_matches_shared_limit() -> None:
+    assert engine_subagent.DEFAULT_MAX_SPAWN_DEPTH == MAX_SPAWN_DEPTH
+
+
+def test_sessions_tool_matches_shared_limit() -> None:
+    assert sessions_tool._MAX_SPAWN_DEPTH == MAX_SPAWN_DEPTH