docs: add CONTRIBUTING.md with dev setup and PR workflow

CONTRIBUTING.md

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+# Contributing to Claude Agent SDK for Python
+
+Thank you for your interest in contributing to the Claude Agent SDK for Python!
+
+## Development Setup
+
+1. Clone the repository:
+
+   ```bash
+   git clone https://github.com/anthropics/claude-agent-sdk-python.git
+   cd claude-agent-sdk-python
+   ```
+
+2. Create and activate a virtual environment (recommended):
+
+   ```bash
+   python -m venv .venv
+   source .venv/bin/activate  # Linux/macOS
+   # or .venv\Scripts\activate  # Windows
+   ```
+
+3. Install the package in editable mode with development dependencies:
+
+   ```bash
+   pip install -e ".[dev]"
+   ```
+
+## Lint and Format
+
+This project uses [Ruff](https://docs.astral.sh/ruff/) for linting and formatting, and [mypy](https://mypy-lang.org/) for static type checking.
+
+```bash
+# Run linter
+ruff check src/ tests/
+
+# Run linter with auto-fix
+ruff check src/ tests/ --fix
+
+# Check formatting
+ruff format --check src/ tests/
+
+# Auto-format code
+ruff format src/ tests/
+
+# Run type checker
+mypy src/
+```
+
+## Testing
+
+Tests are written with [pytest](https://docs.pytest.org/) and use `pytest-asyncio` with `asyncio_mode = "auto"`.
+
+```bash
+# Run all tests
+python -m pytest tests/
+
+# Run tests with coverage
+python -m pytest tests/ --cov=claude_agent_sdk --cov-report=xml
+
+# Run a specific test file
+python -m pytest tests/test_client.py
+```
+
+## Pull Request Guidelines
+
+### Commit Messages and PR Titles
+
+This project follows [Conventional Commits](https://www.conventionalcommits.org/). PR titles and squash-merged commit messages should use one of the following prefixes:
+
+- `feat:` — a new feature
+- `fix:` — a bug fix
+- `docs:` — documentation changes
+- `chore:` — maintenance, dependency bumps, version bumps, etc.
+- `refactor:` — code restructuring without feature or fix changes
+- `perf:` — performance improvements
+- `test:` — adding or updating tests
+- `ci:` — CI configuration changes
+
+Example: `docs: add CONTRIBUTING.md with dev setup instructions`
+
+### Before Submitting
+
+1. Ensure all lint checks pass (`ruff check`, `ruff format --check`, `mypy`).
+2. Ensure all tests pass (`python -m pytest tests/`).
+3. Write a clear PR description explaining the motivation and approach.
+
+## Contributor License Agreement
+
+By submitting a pull request, you agree to the terms of the [Anthropic Contributor License Agreement](https://cla-assistant.io/anthropics/claude-agent-sdk-python). You will be prompted to sign the CLA via the CLA Assistant bot when you open your first pull request.