Add Code Format Check CI (yapf + ruff)

[pandacooming]

Summary

Add GitHub Actions CI for code format checking (yapf + ruff) and a local format.sh script, following the conventions already used in deepseek-ai/DeepEP.

Changes

• .github/workflows/format.yml: GitHub Actions workflow that runs format.sh on every push to main and on every pull request.
• format.sh: Local formatting script that:
  • Runs yapf to format Python files
  • Runs ruff to lint Python files
  • Checks only files changed vs origin/main (or all files with --all)
  • Exits with code 1 if any files need reformatting

Design Decisions

• Python-only: TileKernels contains only .py files, so clang-format is not included (unlike DeepEP which has C/CUDA).
• Ruff version: Pinned to >=0.6.0 (TileKernels pyproject.toml already has a ruff config).
• YAPF version: Pinned to 0.40.2 to match DeepEP.
• CI cache: Uses actions/setup-python built-in pip cache to speed up installs.

Testing

The script was tested locally:

1. Clean state: bash format.sh exits 0 when no files need reformatting ✅
2. New bad formatting: Introducing badly-formatted new code causes the script to exit 1 and list the affected files ✅
3. No forced reformatting: Existing codebase files are not reformatted by this PR (format-on-push only happens after merge)