chore: track checkpoints via Git LFS

Removed checkpoints/*.pt from .gitignore; files are now stored as LFS
objects (pretrained.pt 17 MB, finetuned.pt 17 MB). LFS attributes were
already in place in .gitattributes.

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

.gitignore

@@ -27,10 +27,7 @@ coverage.xml
 .ipynb_checkpoints/
 *.ipynb_checkpoints
 
-# Model checkpoints (large binaries — commit only intentionally)
-checkpoints/*.pt
-checkpoints/*.pth
-checkpoints/*.ckpt
+# Model checkpoints tracked via Git LFS (see .gitattributes)
 
 # Processed data (reproducible from source)
 data/processed/