xk6 is a command-line tool and Go library for building custom k6 binaries with extensions, enabling developers to create, test, and distribute k6 extensions. The tool provides scaffolding for new extensions, a build system that integrates with k6's module system and automatically resolves the correct k6 major version, a linter for extension compliance (including security, licensing, and quality checks), a test runner for k6 scripts, and a dependency synchronizer to keep extension dependencies aligned with k6. It also ships as a Docker image and a Dev Container feature. The codebase is a well-structured Go application using cobra for CLI, with clean separation of concerns across internal packages for linting, scaffolding, sync, and testing. It demonstrates high craftsmanship with proper error handling, logging, and test coverage.