Add CI for linting and type checking

[haix716]

Problem

The project has no CI configuration. While GPU-dependent tests can't run on GitHub Actions free runners, several quality checks can:

• ruff check — Python linting
• mypy — type checking (even partial)
• clang-format --dry-run — C++ style checking

Suggested minimal CI

# .github/workflows/lint.yml
name: Lint
on: [push, pull_request]
jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: astral-sh/ruff-action@v1
      - uses: actions/setup-python@v5
        with: { python-version: "3.12" }
      - run: pip install mypy && mypy moonep/ --ignore-missing-imports

Impact

Quality gate. Catches obvious issues (syntax errors, unused imports, type mismatches) before they reach human review.