feat: generate model card on local save (#258)

[umran666]

I Noticed that if someone hits "Save to local folder", the model card/README gets entirely skipped and only the weights drop.

To fix it without duplicating any code, I just pulled the ModelCard generation logic out of the HF upload block into a clean create_model_card() helper function over in utils.py.
Now the local save routine just hits that exact same helper to automatically drop the README.md right next to the local weight files. Works perfectly and keeps the HF upload logic a lot cleaner too! Let me know if you need anything tweaked.

[gemini-code-assist]

Code Review

This pull request refactors the model card creation logic by introducing a helper function create_model_card in src/heretic/utils.py and using it in src/heretic/main.py both when saving the model locally and when pushing it to the Hugging Face Hub. Feedback was provided to improve the robustness of create_model_card by handling potential exceptions during model card loading and preventing a TypeError when card.text is None.

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