fix: remove large model file from git, add ml/models/ to .gitignore

model.safetensors is 416MB — exceeds GitHub's 100MB limit.
Store locally or use HuggingFace Hub for model weights.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -39,3 +39,4 @@ ml/_smoke_test.py
 
 # Local history persistence (user data — do not commit)
 data/history.json
+ml/models/