docs: add contributing guide

CONTRIBUTING.md

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+# Contributing to Heretic
+
+Thank you for your interest in contributing to Heretic! This document provides guidelines and instructions for contributing.
+
+## Getting Started
+
+### Prerequisites
+
+- Python 3.10 or higher
+- [uv](https://docs.astral.sh/uv/) (recommended for dependency management)
+- A CUDA-capable GPU (for testing model-related changes)
+
+### Setting Up the Development Environment
+
+1. Fork the repository on GitHub.
+
+2. Clone your fork locally:
+
+   ```bash
+   git clone https://github.com/<your-username>/heretic.git
+   cd heretic
+   ```
+
+3. Install dependencies using uv:
+
+   ```bash
+   uv sync --all-extras --dev
+   ```
+
+4. Verify the installation:
+
+   ```bash
+   uv run heretic --help
+   ```
+
+## Development Workflow
+
+### Code Style
+
+This project enforces code style through automated tooling. Please follow these conventions:
+
+- **Formatting**: Code is formatted with [Ruff](https://docs.astral.sh/ruff/). Run `uv run ruff format .` before committing.
+- **Linting**: Imports are sorted and code is linted with Ruff. Run `uv run ruff check --extend-select I .` to check.
+- **Type checking**: All code is type-checked with [ty](https://github.com/astral-sh/ty). Run `uv run ty check .` to verify.
+- **Naming**: Avoid abbreviations in identifier names unless they are widely understood (e.g. "KL divergence").
+- **Comments**: Comments should start with a capital letter and end with a period, using correct grammar and spelling.
+- **Type annotations**: Function and method signatures must be fully type-annotated, including the return type.
+- **SPDX headers**: Every Python source file must start with an SPDX/Copyright header.
+- **Config settings**: When adding new settings in `config.py`, also add them to `config.default.toml` with their default value and description as a comment. The order in `config.default.toml` should match `config.py`.
+
+### Running Checks
+
+Before submitting a pull request, ensure all checks pass locally:
+
+```bash
+# Format check
+uv run ruff format --check .
+
+# Lint and import sort check
+uv run ruff check --output-format=github --extend-select I .
+
+# Type check
+uv run ty check --output-format=github --error-on-warning .
+
+# Build verification
+uv build
+```
+
+The CI pipeline runs these checks on Python 3.10, 3.11, 3.12, and 3.13.
+
+### Making Changes
+
+1. Create a new branch from `master`:
+
+   ```bash
+   git checkout -b <branch-name> master
+   ```
+
+2. Make your changes, following the code style guidelines above.
+
+3. Commit your changes with a descriptive message.
+
+4. Push to your fork and open a pull request against the `master` branch.
+
+## Pull Request Guidelines
+
+- **One change per PR**: Each pull request should implement one change, and one change only. If your changes are semantically independent, split them into separate PRs.
+- **Do not modify unrelated code**: Do not change existing code unless the changes are directly related to the PR. This includes formatting and comment changes to unrelated files.
+- **Semantic PR title**: PR titles are validated by CI. Use a [Conventional Commits](https://www.conventionalcommits.org/) prefix:
+  - `feat:` for new features
+  - `fix:` for bug fixes
+  - `perf:` for performance improvements
+  - `docs:` for documentation changes
+  - `build:` for build system changes
+- **Describe your changes**: Include a summary of what your PR does and why in the PR description.
+- **Link related issues**: If your PR addresses an issue, reference it in the description (e.g. "Closes #123").
+
+## Reporting Issues
+
+- Search [existing issues](https://github.com/p-e-w/heretic/issues) before opening a new one.
+- Include your environment details (OS, Python version, GPU, PyTorch version) when reporting bugs.
+- Provide the full error output and steps to reproduce the problem.
+
+## License
+
+By contributing to this project, you agree to release your contributions under the [AGPL-3.0-or-later](LICENSE) license.