Add a GitHub Actions workflow that validates marketplace.json is
well-formed JSON with a plugins array whenever PRs modify it. Includes:
- validate-marketplace.ts: Bun script that parses and validates the JSON
- validate-marketplace.yml: GH Actions workflow triggered on PR changes
- test-marketplace-check.js: Unit tests for the validation logic
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
