docs: add CONTRIBUTING.md with dev setup, lint/test commands, and PR conventions

CONTRIBUTING.md

@@ -0,0 +1,55 @@
+# Contributing
+
+Thanks for contributing to the Claude Agent SDK for Python!
+
+## Setting up your environment
+
+Install the package in editable mode along with the development dependencies (pytest, mypy, ruff, and friends):
+
+```bash
+pip install -e ".[dev]"
+```
+
+Then install the git hooks:
+
+```bash
+./scripts/initial-setup.sh
+```
+
+This installs a pre-push hook that runs the same lint checks as CI before each push. To skip the hook temporarily, use `git push --no-verify`.
+
+## Linting and formatting
+
+```bash
+# Check for issues and fix automatically
+python -m ruff check src/ tests/ --fix
+
+# Format code
+python -m ruff format src/ tests/
+```
+
+CI (and the pre-push hook) runs `python -m ruff check src/ tests/` and `python -m ruff format --check src/ tests/`, so both must pass cleanly.
+
+## Typechecking
+
+Typechecking is only enforced for `src/`:
+
+```bash
+python -m mypy src/
+```
+
+## Running tests
+
+```bash
+# Run all tests
+python -m pytest tests/
+
+# Run a specific test file
+python -m pytest tests/test_client.py
+```
+
+## Pull requests
+
+PR titles follow the [Conventional Commits](https://www.conventionalcommits.org/) format, e.g. `fix: handle empty errors list in result messages` or `docs: document dev setup`. Common types used in this repo are `feat`, `fix`, `docs`, `chore`, and `ci`.
+
+Before opening a PR, make sure lint, typecheck, and tests all pass locally.

README.md

@@ -289,13 +289,14 @@ If you're upgrading from the Claude Code SDK (versions < 0.1.0), please see the
 
 ## Development
 
-If you're contributing to this project, run the initial setup script to install git hooks:
+If you're contributing to this project, see [CONTRIBUTING.md](CONTRIBUTING.md) for dev environment setup, lint/typecheck/test commands, and PR conventions. In short:
 
 ```bash
-./scripts/initial-setup.sh
+pip install -e ".[dev]"   # install dev dependencies
+./scripts/initial-setup.sh  # install git hooks
 ```
 
-This installs a pre-push hook that runs lint checks before pushing, matching the CI workflow. To skip the hook temporarily, use `git push --no-verify`.
+The setup script installs a pre-push hook that runs lint checks before pushing, matching the CI workflow. To skip the hook temporarily, use `git push --no-verify`.
 
 ### Building Wheels Locally