Add pre-commit config and CONTRIBUTING.md

[Larslllllll]

Summary

• Add .pre-commit-config.yaml with ruff + basic hooks (trailing-whitespace, end-of-file-fixer)
• Add CONTRIBUTING.md with development workflow documentation

Acceptance Criteria

• .pre-commit-config.yaml (ruff, trailing-whitespace, eof)
• Docs in CONTRIBUTING

Closes #113

[codeforstartups]

Thanks @Larslllllll — the pre-commit config and CONTRIBUTING.md are excellent and I'd love to merge them. Two things to fix first:

1. pip-audit step is misconfigured and would fail CI on development:
   • pypa/gh-action-pip-audit has no requirements: input, and pyproject.toml isn't a requirements file.
   • Use the local-project mode instead:

       - name: Run pip-audit
         uses: pypa/gh-action-pip-audit@v1.1.0
         with:
           inputs: .

2. Overlap with #118 — both PRs make the identical ci.yml change (add 3.13 + a security job). Let's land the ci.yml change in exactly one PR. Since this PR also brings pre-commit + CONTRIBUTING, I'd suggest this one own it, and we close #118 (or trim #118 to nothing).

Once the pip-audit step is fixed I'll merge. Really nice contribution!

[codeforstartups]

👋 The pre-commit config + CONTRIBUTING.md are ready to merge — the only blocker is the same pip-audit step:

      - name: Run pip-audit
        uses: pypa/gh-action-pip-audit@v1.1.0
        with:
          inputs: .        # not: requirements: pyproject.toml

Fix that (or drop the security job into its own follow-up PR) and I'll merge this. If you'd rather ship the docs/pre-commit now, remove the ci.yml hunk here and I'll merge immediately.

[codeforstartups]

Superseded by #160 (now merged), which adds the pre-commit config + CONTRIBUTING.md without the broken pip-audit CI job. Thank you @Larslllllll for kicking this off — closing in favor of #160. The 3.13 matrix from your #118 lives on in #158.