Formalize OEIS A147983 Chomp challenge

[DomTheDeveloper]

Superseded by #134. The replacement branch is based directly on current Google DeepMind Formal Conjectures main, follows the numeric OEIS filename and namespace conventions, and preserves the required two-commit history.

[github-actions]

👋 This is an automated welcome message. 🤖
Thanks for the contributions!

A few friendly reminders while the review gets started:

• Please take a look at the style guidelines,
  especially the conventions for references, categories, AMS tags, and answer(sorry).
• You can manage some PR labels by leaving a comment with +label-name or -label-name; for example, +awaiting-author or -awaiting-author.
• This repository is mainly for formalised statements. Proofs longer than about 25-50 lines are usually out of scope; longer proofs are welcome to be included/linked via the formal_proof mechanism.

Thanks again for helping improve Formal Conjectures.