Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Setup
+
+Use [`nvm`](https://github.com/nvm-sh/nvm) to manage Node versions. CI runs on Node 20; anything >= 18 works.
+
+```bash
+nvm install 20
+nvm use 20
+```
+
+This repo uses Yarn (Berry, via `.yarnrc.yml`). Install dependencies:
+
+```bash
+yarn
+```
+
+## Development
+
+Generated GraphQL clients live in `generated/`. Regenerate them after changing queries or the schema:
+
+```bash
+yarn gen-clients
+```
+
+Build both CJS and ESM outputs:
+
+```bash
+yarn build
+```
+
+## Testing and linting
+
+```bash
+yarn test        # vitest, watch mode
+yarn test:ci     # single run
+yarn lint
+yarn typecheck
+```
+
+To refresh the schema used by tests, run `yarn update-test-schema`.
+
+## Docs
+
+```bash
+yarn gen-docs
+yarn start-docs
+```
+
+## Releasing
+
+Publishing is handled by GitHub Actions (`.github/workflows/publish.yml`). Bump the version with `yarn version`, which also regenerates `generated/version.ts` via the `postversion` hook.