Add comprehensive CONTRIBUTING.md guide for new contributors

CONTRIBUTING.md

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+# Contributing to malariagen-data-python
+
+Thanks for your interest in contributing to this project! This guide will help you get started.
+
+## About the project
+
+This package provides Python tools for accessing and analyzing genomic data from [MalariaGEN](https://www.malariagen.net/), a global research network studying the genomic epidemiology of malaria and its vectors. It provides access to data on _Anopheles_ mosquito species and _Plasmodium_ malaria parasites, with functionality for variant analysis, haplotype clustering, population genetics, and visualization.
+
+## Setting up your development environment
+
+### Prerequisites
+
+You'll need:
+
+- Python 3.10.x (CI-tested version)
+- [Poetry](https://python-poetry.org/) for dependency management
+- [Git](https://git-scm.com/) for version control
+
+### Initial setup
+
+1. **Fork and clone the repository**
+
+   Fork the repository on GitHub, then clone your fork:
+
+   ```bash
+   git clone git@github.com:[your-username]/malariagen-data-python.git
+   cd malariagen-data-python
+   ```
+
+2. **Add the upstream remote**
+
+   ```bash
+   git remote add upstream https://github.com/malariagen/malariagen-data-python.git
+   ```
+
+3. **Install Poetry** (if not already installed)
+
+   ```bash
+   pipx install poetry
+   ```
+
+4. **Install the project and its dependencies**
+
+   ```bash
+   poetry install
+   ```
+
+   **Recommended**: Use `poetry run` to run commands inside the virtual environment:
+
+   ```bash
+   poetry run pytest
+   poetry run python script.py
+   ```
+
+   **Optional**: If you prefer an interactive shell session, install the shell plugin first:
+
+   ```bash
+   poetry self add poetry-plugin-shell
+   ```
+
+   Then activate the environment with:
+
+   ```bash
+   poetry shell
+   ```
+
+   After activation, commands run directly inside the virtual environment:
+
+   ```bash
+   pytest
+   python script.py
+   ```
+
+5. **Install pre-commit hooks**
+
+   ```bash
+   pipx install pre-commit
+   pre-commit install
+   ```
+
+   Pre-commit hooks will automatically run `ruff` (linter and formatter) on your changes before each commit.
+
+## Development workflow
+
+### Creating a new feature or fix
+
+1. **Sync with upstream**
+
+   ```bash
+   git checkout master
+   git pull upstream master
+   ```
+
+2. **Create a feature branch**
+
+   If an issue does not already exist for your change, [create one](https://github.com/malariagen/malariagen-data-python/issues/new) first. Then create a branch using the convention `GH{issue number}-{short description}`:
+
+   ```bash
+   git checkout -b GH123-fix-broken-filter
+   # or
+   git checkout -b GH456-add-new-analysis
+   ```
+
+3. **Make your changes**
+
+   Write your code, add tests, update documentation as needed.
+
+4. **Run tests locally**
+
+   Fast unit tests (no external data access):
+
+   ```bash
+   poetry run pytest -v tests/anoph
+   ```
+
+   All unit tests (requires setting up credentials for legacy tests):
+
+   ```bash
+   poetry run pytest -v tests --ignore tests/integration
+   ```
+
+5. **Check code quality**
+
+   The pre-commit hooks will run automatically, but you can also run them manually:
+
+   ```bash
+   pre-commit run --all-files
+   ```
+
+### Code style
+
+We use `ruff` for both linting and formatting. The configuration is in `pyproject.toml`. Key points:
+
+- Line length: 88 characters (black default)
+- Follow PEP 8 conventions
+- Use type hints where appropriate
+- Write clear docstrings (we use numpydoc format)
+
+The pre-commit hooks will handle most formatting automatically. If you want to run ruff manually:
+
+```bash
+ruff check .
+ruff format .
+```
+
+### Testing
+
+- **Write tests for new functionality**: Add unit tests in the `tests/` directory
+- **Test coverage**: Aim to maintain or improve test coverage
+- **Fast tests**: Unit tests should use simulated data when possible (see `tests/anoph/`)
+- **Integration tests**: Tests requiring GCS data access are slower and run separately
+
+Run type checking with:
+
+```bash
+poetry run pytest -v tests --typeguard-packages=malariagen_data,malariagen_data.anoph
+```
+
+### Documentation
+
+- Update docstrings if you modify public APIs
+- Documentation is built using Sphinx with the pydata theme
+- API docs are auto-generated from docstrings
+- Follow the [numpydoc](https://numpydoc.readthedocs.io/) style guide
+
+## Submitting your contribution
+
+### Before opening a pull request
+
+- [ ] Tests pass locally
+- [ ] Pre-commit hooks pass (or run `pre-commit run --all-files`)
+- [ ] Code is well-documented
+- [ ] Commit messages are clear and descriptive
+
+### Opening a pull request
+
+1. **Push your branch**
+
+   ```bash
+   git push origin your-branch-name
+   ```
+
+2. **Create the pull request**
+   - Go to the [repository on GitHub](https://github.com/malariagen/malariagen-data-python)
+   - Click "Pull requests" → "New pull request"
+   - Select your fork and branch
+   - Write a clear title and description
+
+3. **Pull request description should include:**
+   - What problem does this solve?
+   - How does it solve it?
+   - Any relevant issue numbers (e.g., "Fixes #123")
+   - Testing done
+   - Any breaking changes or migration notes
+
+### Review process
+
+- PRs require approval from a project maintainer
+- CI tests must pass (pytest on Python 3.10 with NumPy 1.26.4)
+- Address review feedback by pushing new commits to your branch
+- Once approved, a maintainer will merge your PR
+
+## AI-assisted contributions
+
+We welcome contributions that involve AI tools (like GitHub Copilot, ChatGPT, or similar). If you use AI assistance:
+
+- Review and understand any AI-generated code before submitting
+- Ensure the code follows project conventions and passes all tests
+- You remain responsible for the quality and correctness of the contribution
+- Disclosure of AI usage is optional. Regardless of tools used, contributors remain responsible for the quality and correctness of their submissions.
+
+## Communication
+
+- **Issues**: Use [GitHub Issues](https://github.com/malariagen/malariagen-data-python/issues) for bug reports and feature requests
+- **Discussions**: For questions and general discussion, use [GitHub Discussions](https://github.com/malariagen/malariagen-data-python/discussions)
+- **Pull requests**: Use PR comments for code review discussions
+- **Email**: For data access questions, contact [support@malariagen.net](mailto:support@malariagen.net)
+
+## Finding something to work on
+
+- Look for issues labeled [`good first issue`](https://github.com/malariagen/malariagen-data-python/labels/good%20first%20issue)
+- Check for issues labeled [`help wanted`](https://github.com/malariagen/malariagen-data-python/labels/help%20wanted)
+- Improve documentation or add examples
+- Increase test coverage
+
+## Questions?
+
+If you're unsure about anything, feel free to:
+
+- Open an issue to ask
+- Start a discussion on GitHub Discussions
+- Ask in your pull request
+
+We appreciate your contributions and will do our best to help you succeed!
+
+## License
+
+By contributing to this project, you agree that your contributions will be licensed under the [MIT License](LICENSE).