Add 'run' key to cog.yaml alongside legacy 'predict' key
- Accept both 'run:' and 'predict:' in cog.yaml (cannot set both)
- 'run' takes precedence; 'predict' is the legacy name
- Update JSON schema to include 'run' property
- Update cog init template: generate run.py with BaseRunner class
- Update cog.yaml template to use run: 'run.py:Runner'
- Add validation tests for run/predict conflict
