docs: add CONTRIBUTING.md with dev setup and workflow

CONTRIBUTING.md

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+# Contributing to claude-agent-sdk-python
+
+Thank you for your interest in contributing! This guide covers how to set up
+your development environment and the workflow for submitting changes.
+
+## Development Setup
+
+**Requirements:** Python 3.10+
+
+Clone the repository and install the package with its dev dependencies:
+
+```bash
+git clone https://github.com/anthropics/claude-agent-sdk-python.git
+cd claude-agent-sdk-python
+pip install -e ".[dev]"
+```
+
+If you prefer [uv](https://github.com/astral-sh/uv):
+
+```bash
+uv pip install -e ".[dev]"
+```
+
+The `dev` extra installs: `pytest`, `pytest-cov`, `anyio[trio]`, `mypy`, and `ruff`.
+
+## Running Tests
+
+```bash
+python -m pytest tests/
+```
+
+Run with coverage:
+
+```bash
+python -m pytest tests/ --cov=src/claude_agent_sdk
+```
+
+## Lint and Style
+
+Check for issues and auto-fix where possible:
+
+```bash
+python -m ruff check src/ tests/ --fix
+python -m ruff format src/ tests/
+```
+
+## Type Checking
+
+```bash
+python -m mypy src/
+```
+
+Mypy is configured in `pyproject.toml` with strict mode enabled.
+
+## Submitting a Pull Request
+
+1. Fork the repository and create a branch from `main`:
+   ```bash
+   git checkout main -b fix/your-short-description
+   ```
+2. Make your changes. Keep each PR focused on a single fix or feature.
+3. Ensure tests pass, lint is clean, and mypy reports no errors.
+4. Write a clear commit message that describes *what* changed and *why*.
+5. Open a pull request against `anthropics/claude-agent-sdk-python:main`.
+   Fill in the PR description with context on the problem being solved.
+
+Please keep PRs small and focused — one fix or feature per PR makes review
+faster and merges cleaner.