Add contributing guide

[ghostwriternr]

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Latest commit: b7046e6

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[claude]

Claude Code Review

Issue: Broken CONTRIBUTING.md link in README

The README.md link to CONTRIBUTING.md (line 121) is incorrect:

See [CONTRIBUTING.md](./CONTRIBUTING.md) for guidelines

This path won't work because CONTRIBUTING.md is at the repository root, but the README is in packages/sandbox/. The link should be:

See [CONTRIBUTING.md](../../CONTRIBUTING.md) for guidelines

Node version mismatch

CONTRIBUTING.md:9 states "Node.js 24+" but the project actually uses Node.js 20 LTS according to CLAUDE.md ("Node.js 20 LTS"). The workflows also use node-version: 24. This should be clarified - either the project has upgraded to Node 24 (update CLAUDE.md) or CONTRIBUTING.md should say Node 20+ (match current reality).

Otherwise looks good

The CI workflow changes properly gate external PRs with the 'ci' label while allowing internal PRs to run automatically - good security practice. The CONTRIBUTING guide is well-structured and helpful.

[github-actions]

🐳 Docker Image Published

FROM cloudflare/sandbox:0.0.0-pr-179-50121a7

Version: 0.0.0-pr-179-50121a7

You can use this Docker image with the preview package from this PR.