docs: add CONTRIBUTING.md with dev setup, lint, test, and PR guidelines

CONTRIBUTING.md

@@ -0,0 +1,48 @@
+# Contributing to claude-agent-sdk
+
+## Development Setup
+
+1. Clone the repo and install with dev dependencies:
+
+```bash
+pip install -e ".[dev]"
+```
+
+2. Install the pre-push git hook (runs lint checks before push, matching CI):
+
+```bash
+./scripts/initial-setup.sh
+```
+
+To skip the hook temporarily: `git push --no-verify`.
+
+## Lint & Format
+
+```bash
+python -m ruff check src/ tests/ --fix
+python -m ruff format src/ tests/
+```
+
+## Type Checking
+
+```bash
+python -m mypy src/
+```
+
+Type checking is only enforced for `src/` (not `tests/`).
+
+## Tests
+
+```bash
+# All tests
+python -m pytest tests/
+
+# Single test file
+python -m pytest tests/test_client.py
+```
+
+## PR Guidelines
+
+- Use [Conventional Commits](https://www.conventionalcommits.org/) for PR titles (e.g., `fix:`, `feat:`, `docs:`, `chore:`).
+- Keep PRs focused on a single change.
+- Make sure lint, typecheck, and tests pass before opening the PR.

README.md

@@ -289,6 +289,9 @@ If you're upgrading from the Claude Code SDK (versions < 0.1.0), please see the
 
 ## Development
 
+See [CONTRIBUTING.md](CONTRIBUTING.md) for the full development setup, including dev-dependency install, lint, typecheck, and test commands.
+
+
 If you're contributing to this project, run the initial setup script to install git hooks:
 
 ```bash