fix(ci): download model from Hugging Face instead of storing in repo

- Remove binary model artifacts tracked in repo (.tflite, saved_model, tfjs model.json)
- CI now downloads best_model.keras from Hugging Face Hub (MythEclipse2737/corn-leaf-disease-classifier)
- Add save_model.py step before convert_onnx.py in CI pipeline
- Remove LFS patterns from .gitattributes (no longer needed)
- Remove lfs:true from checkout action

The model is fetched at CI time using HF_TOKEN secret — no large
binary files stored in git.

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

Machine_Learning/model/saved_model/fingerprint.pb

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