Fix DVC output path overlap in train stage

Changed from tracking entire models/ directory to specific model file
to resolve conflict with models/metrics.json metric tracking.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

dvc.yaml

@@ -26,7 +26,7 @@ stages:
       - train.max_depth
       - train.random_state
     outs:
-      - models/
+      - models/multitask_model.pkl
     metrics:
       - models/metrics.json:
           cache: false