Add classifier_model and feedback_model support to YAML schema

Allow activities to specify separate models for classification and feedback:
- classifier_model: Used for categorizing user responses into buckets
- feedback_model: Used for generating AI feedback and translations

Both fields can be set at activity level (defaults) and overridden at step level.

Updated activity37 to use:
- MODEL_1 (Hermes) for classification
- MODEL_3 (Qwen 3 Coder) for feedback

This allows using specialized models for different tasks, e.g., fast classification
with accurate feedback generation from domain-specific models.

research/activity37-programming-languages.yaml

@@ -1,5 +1,11 @@
 default_max_attempts_per_step: 3
 
+# Model configuration
+# Use Hermes for classification (fast, accurate bucketing)
+# Use Qwen 3 Coder for feedback (specialized for code generation)
+classifier_model: "MODEL_1"
+feedback_model: "MODEL_3"
+
 tokens_for_ai_rubric: |
   Evaluate the student's understanding of programming concepts in their chosen language.
   Consider: