Add mojo format to lint pipeline; parallelize update_challenges.py uploads

[kunal-mansukhani]

Summary

• Wires mojo format into pre-commit and the lint-mojo CI job, and reformats all 78 existing .mojo starter files so the check goes green.
• Adds modular (provides the mojo binary) to the challenge-creation cron's pip install, so Claude's pre-commit run --all-files step won't fail on missing mojo.
• Drops target-version = ['py312'] from [tool.black] in pyproject.toml: mojo format wraps mblack, which reads the same [tool.black] section but only accepts targets up to py311 (or mojo). Black auto-detects the target fine without it.
• Parallelizes scripts/update_challenges.py: HTTP uploads now fan out via ThreadPoolExecutor(max_workers=16). Loading stays sequential because importlib reuses a "challenge" module name and mutates sys.path, which isn't thread-safe.

Test plan

• pre-commit run --all-files — all hooks pass locally (black, isort, flake8, clang-format, mojo format, etc.)
• mojo format -q run over all challenges/**/*.mojo; git diff --exit-code clean afterward
• python -c "import ast; ast.parse(open('scripts/update_challenges.py').read())" parses cleanly
• Verify the lint-mojo CI job installs modular and the format check passes on this PR
• Verify update_challenges.py still deploys correctly end-to-end against staging (sanity check that concurrent POSTs don't hit a rate limit — max_workers=16 may need tuning)
• Next scheduled run of the challenge-creation cron succeeds with mojo now available on PATH

🤖 Generated with Claude Code