test(block-h): gate H proof - plugin contributes all extension points without core changes

app/workflows/step_handlers.py

@@ -61,6 +61,11 @@ def register_step_type(step_type: str):
     return decorator
 
 
+def unregister_step_type(step_type: str) -> None:
+    """Remove a step handler (plugin deactivation lifecycle symmetry)."""
+    _HANDLERS.pop(step_type, None)
+
+
 def get_step_handler(step_type: str) -> StepHandler | None:
     return _HANDLERS.get(step_type)