Fix: Use correct GPT model config (open_model.yaml)

Changes config from open_model_v0.5.yaml to open_model.yaml to match the actual checkpoint architecture (23 layers, no embedder layers).

This resolves the AssertionError caused by config/checkpoint version mismatch.

code/demo.py

@@ -57,7 +57,7 @@ def get_gpt_engine_cached():
     print("🔧 Initializing GPT engine (one-time setup)...")
 
     # Use absolute path relative to this file (fixes HF Spaces deployment)
-    config_path = os.path.join(os.path.dirname(__file__), 'cube3d/configs/open_model_v0.5.yaml')
+    config_path = os.path.join(os.path.dirname(__file__), 'cube3d/configs/open_model.yaml')
     gpt_ckpt_path = None  # test mode doesn't use this
 
     # Detect HuggingFace Spaces environment