Add challenge structure validator

[this-AR]

Summary

Adds a reusable challenge validation script and wires it into CI.

The new scripts/validate_challenges.py checks challenge structure without importing runtime dependencies like PyTorch, CUDA, or JAX. It uses Python AST parsing to validate challenge metadata and required methods, making it safe to run in lightweight CI environments.

What Changed

• Added scripts/validate_challenges.py
• Replaced the inline duplicate-index GitHub Action logic with the validator
• Documented the validation command in CONTRIBUTING.md
• Cleaned up minor README / CONTRIBUTING encoding and formatting issues

Validation Covered

The script checks:

• required files: challenge.py, challenge.html
• duplicate challenge IDs
• challenge directory naming
• required Challenge metadata fields
• required challenge methods
• supported starter filenames
• missing starter templates as warnings by default

Testing

Ran locally:

python scripts/validate_challenges.py
python -m black --check --diff scripts/validate_challenges.py
python -m py_compile scripts/validate_challenges.py
git diff --check