feat: Add model loader system (same as prompts)

New structure:
- models/standard/*.py: Default models (XGBoost, LightGBM, RandomForest)
- models/local/*.py: Your improved models (NOT in Git!)
- models/README.md: Documentation
- rdagent/components/model_loader.py: Model loader with priority

Features:
- Loader checks models/local/ first (your better models)
- Falls back to models/standard/ if no local version
- Supports versioned models (model_v2.py, model_v1.py)
- Lists available models
- Test function included

.gitignore updated:
- models/local/ excluded (your proprietary models)
- *.local.py excluded
- *_private.py excluded

Usage:
  from rdagent.components.model_loader import load_model
  model = load_model('xgboost_factor')  # Auto-loads your better version!

Standard models included:
- xgboost_factor.py: XGBoost for tabular data
- lightgbm_factor.py: LightGBM (faster than XGBoost)

.gitignore

@@ -87,6 +87,11 @@ prompts/local/
 *.local.yaml
 *_private.yaml
 
+# Private models (your improved versions)
+models/local/
+*.local.py
+*_private.py
+
 # Test credentials
 .env.test
 *.test.env