docs: add CONTRIBUTING.md with lint/format instructions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: FrancescoSaverioZuppichini

CONTRIBUTING.md

@@ -0,0 +1,31 @@
+# Contributing
+
+## Setup
+
+```bash
+uv sync
+```
+
+## Before committing
+
+```bash
+uv run ruff check src/ tests/   # lint
+uv run ruff format src/ tests/  # format
+uv run pytest tests/test_client.py -v  # tests
+```
+
+Or all at once:
+
+```bash
+uv run ruff check src/ tests/ && uv run ruff format src/ tests/ && uv run pytest tests/test_client.py -v
+```
+
+CI will reject PRs that fail lint or format checks.
+
+## Running integration tests
+
+Requires `SGAI_API_KEY` env var:
+
+```bash
+uv run pytest tests/test_integration.py -v
+```