feat: add free distillable models

[realityinspector]

Summary

• Adds three free distillable models to MODEL_REGISTRY: Hunter Alpha (1M context), Healer Alpha (262K, multimodal), and Nemotron 3 Super 120B (262K, strong reasoning)
• All three have relative_cost=0.0 and training_data_unrestricted=True — zero cost with full distillation rights
• Adds select_free_distillable() method to ModelSelector for selecting the best free model by quality
• Adds free_distillable config section to conf/config.yaml
• Fixes potential ZeroDivisionError in cost scoring for zero-cost models

Test plan

• Verify ModelSelector().select_free_distillable(ActionType.DIALOG_SYNTHESIS) returns nvidia/nemotron-3-super-120b-a12b:free (highest quality=0.95)
• Verify get_training_safe_models() includes all three new models
• Verify select_model(ActionType.DIALOG_SYNTHESIS, prefer_cost=True) does not raise ZeroDivisionError
• Confirm config loads cleanly with free_distillable section