feat: add pre-commit configuration and contributing guide

[Ashishds]

Closes #113

Summary of Changes

• Added .pre-commit-config.yaml configured with:
  • ruff and ruff-format
  • Standard hygiene hooks (trailing-whitespace, end-of-file-fixer, check-yaml, check-toml)
• Added pre-commit to the dev extra in pyproject.toml.
• Added a comprehensive CONTRIBUTING.md guide covering:
  • Local setup (clone, virtual environment, editable install)
  • Installing and running pre-commit hooks
  • Running test suite and linter checks (ruff, pytest)
  • Branching and Conventional Commits PR guidelines
• Formatted test files to ensure full compliance with newly introduced hooks.

Local Verification

• Verified pre-commit run --all-files passes cleanly across all tracked files.
• Ran full test suite: pytest passed (127 passed, 2 skipped).
• Ran linter: ruff check src benchmarks tests passed with zero errors.

[codeforstartups]

Verified (tests + ruff green). pre-commit + CONTRIBUTING with no broken CI job — supersedes #117. Thanks @Ashishds!