✨ Set up Copilot instructions for EasyReflectometryLib

.github/copilot-instructions.md

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+# GitHub Copilot Instructions for EasyReflectometryLib
+
+## Project Overview
+
+EasyReflectometryLib is a reflectometry Python package built on the EasyScience framework. It provides tools for reflectometry analysis and modeling.
+
+## Development Environment
+
+- **Python Versions**: 3.11, 3.12
+- **Supported Platforms**: Linux (ubuntu-latest), macOS (macos-latest), Windows (windows-latest)
+- **Package Manager**: pip
+- **Build System**: hatchling with setuptools-git-versioning
+
+## Code Style and Formatting
+
+### Ruff Configuration
+- Use **Ruff** for linting and formatting (configured in `pyproject.toml`)
+- Maximum line length: 127 characters
+- Quote style: single quotes for strings
+- Import style: force single-line imports
+- To fix issues automatically: `python -m ruff . --fix`
+
+### Code Quality Standards
+- Follow PEP 8 guidelines
+- Use type hints where appropriate
+- Write clear, self-documenting code with meaningful variable names
+- Maintain consistency with existing code patterns in the repository
+
+### Linting Rules
+The project uses Ruff with the following rule sets:
+- `E9`, `F63`, `F7`, `F82`: Critical flake8 rules
+- `E`: pycodestyle errors
+- `F`: Pyflakes
+- `I`: isort (import sorting)
+- `S`: flake8-bandit (security checks)
+
+Special notes:
+- Asserts are allowed in test files (`*test_*.py`)
+- Init module imports are ignored
+- Exclude `docs` directory from linting
+
+## Testing
+
+### Test Framework
+- Use **pytest** for all tests
+- Test coverage should be tracked with **pytest-cov**
+- Aim for comprehensive test coverage
+- Tests are located in the `tests/` directory
+
+### Running Tests
+```bash
+# Install dev dependencies
+pip install -e '.[dev]'
+
+# Run tests with coverage
+pytest --cov --cov-report=xml
+
+# Run tests using tox (for multiple Python versions)
+pip install tox tox-gh-actions
+tox
+```
+
+### Test Guidelines
+- Write unit tests for all new functionality
+- Include tests when fixing bugs to prevent regression
+- Test files should match the pattern `test_*.py`
+- Use descriptive test function names that explain what is being tested
+- Follow the existing test structure and patterns in the repository
+
+## Security
+
+- Follow flake8-bandit security guidelines (enabled via Ruff `S` rules)
+- Be cautious with user input and file operations
+- Do not commit secrets or sensitive information
+- Review security implications of all changes
+
+## Documentation
+
+### Docstring Style
+- Include docstrings for all public modules, classes, and functions
+- Use **Sphinx/reStructuredText style** docstrings (`:param`, `:type`, `:return`, `:rtype`)
+- Use clear, concise descriptions
+- Document parameters, return values, and exceptions
+- Example format:
+  ```python
+  """
+  Brief description of the function.
+
+  :param param_name: description of parameter
+  :type param_name: type
+  :return: description of return value
+  :rtype: return_type
+  """
+  ```
+
+### Documentation Build
+- Documentation is built using Sphinx (version 8.1.3)
+- Source files are in the `docs/` directory
+- Use `myst_parser` (MyST parser) for Markdown support
+- Include code examples in documentation where appropriate
+
+## Dependencies
+
+### Core Dependencies
+- easyscience (EasyScience framework)
+- scipp (Scientific computing)
+- refnx, refl1d (Reflectometry calculations)
+- orsopy (Data format support)
+- bumps (Optimization)
+
+### Adding New Dependencies
+- Only add dependencies when absolutely necessary
+- Add to appropriate section in `pyproject.toml`:
+  - `dependencies` for core runtime dependencies
+  - `dev` for development tools
+  - `docs` for documentation building
+- Document why the dependency is needed
+
+## Git and Version Control
+
+### Commit Messages
+- Write clear, descriptive commit messages
+- Use present tense ("Add feature" not "Added feature")
+- Reference issue numbers when applicable
+
+### Branch Workflow
+- Create feature branches from the main branch
+- Use descriptive branch names (e.g., `feature/add-new-calculator`, `bugfix/fix-reflection-calculation`)
+- Keep changes focused and atomic
+
+## Pull Request Guidelines
+
+1. Include tests for new functionality
+2. Update documentation if adding or changing features
+3. Ensure all CI checks pass:
+   - Code consistency (Ruff)
+   - Code testing (pytest on all supported platforms/versions)
+   - Package building
+4. Code should work on Python 3.11, 3.12 and all supported platforms
+5. Write a clear PR description explaining the changes
+
+## Project Structure
+
+```
+src/easyreflectometry/     # Main package source code
+├── calculators/           # Calculator implementations (refnx, refl1d)
+│   └── bornagain/         # BornAgain calculator (not yet functional)
+├── model/                 # Reflectometry models
+├── sample/                # Sample structures and materials
+├── special/               # Special calculations and parsing
+├── summary/               # Summary generation
+└── project.py             # Main project interface
+
+tests/                     # Test suite
+docs/                      # Documentation source
+```
+
+## Best Practices
+
+1. **Minimal Changes**: Make the smallest possible changes to accomplish the task
+2. **Don't Break Existing Code**: Maintain backward compatibility unless explicitly required
+3. **Test Before Committing**: Always run tests and linting before pushing
+4. **Follow Existing Patterns**: Look at similar code in the repository for guidance
+5. **Ask When Uncertain**: If unsure about an approach, ask for clarification
+
+## CI/CD Pipeline
+
+The project uses GitHub Actions for continuous integration:
+- **Code Consistency**: Runs Ruff linting on all pushes and PRs
+- **Code Testing**: Runs pytest across multiple Python versions and platforms
+- **Package Testing**: Validates package building and installation
+- **Coverage**: Uploads test coverage to Codecov
+
+All CI checks must pass before merging PRs.
+
+## Special Notes
+
+- The project is part of the EasyScience ecosystem
+- Built on top of established reflectometry libraries (refnx, refl1d)
+- Focuses on providing a user-friendly interface for reflectometry analysis
+- Maintains compatibility with multiple calculator backends