docs(frontierchallenge): add arXiv citation

[fzp0424]

Summary

• replace the FrontierChallenge BibTeX entry with the official author order and arXiv metadata
• add the direct arXiv URL for paper and dataset association
• align the GitHub citation with both Hugging Face dataset cards

Hugging Face

Updated and verified:

• apodex/FrontierChallenge
• apodex/FrontierChallenge-reference

Both remote dataset READMEs contain https://arxiv.org/abs/2608.24979.

Validation

• git diff --check
• one commit and one changed file
• remote HF README hashes match the uploaded files