Add HF_TOKEN support and improve gated repo error messages

[TroyHernandez]

Summary

• Add HF_TOKEN as the primary environment variable for authentication (it's the standard used by HuggingFace CLI and Python libraries)
• Provide helpful error messages for gated repos and auth failures

Changes

1. HF_TOKEN support: Check HF_TOKEN first, then fall back to HUGGING_FACE_HUB_TOKEN and HUGGINGFACE_HUB_TOKEN

2. Better error messages:

   • GatedRepo (403): Instructs user to visit the model page and accept the license
   • Unauthorized (401): Instructs user to set HF_TOKEN and provides link to token settings
   • Other errors: Shows the error message from HuggingFace API

Example error output

Before:

Error: Distant resource does not seem to be on huggingface.co (missing commit header).

After:

Error: Access denied to gated repository.
ℹ This model requires accepting a license agreement.
ℹ Visit <https://huggingface.co/google/gemma-3-12b-it> to accept the terms.
ℹ Make sure you are logged in with the account that owns your HF_TOKEN.

Testing

Tested with google/gemma-3-12b-it (a gated model requiring license acceptance).