test: add npm tarball smoke test

[luojiyin1987]

Summary

• upgrade @lint-md/core from ^2.1.5 to ^2.1.6 for parser 0.2.0 source-map fixes and accurate inline-code value ranges
• add an npm tarball smoke test that installs the packed CLI into an isolated global prefix
• verify the package contents, installed executable, stdin linting, worker-backed file linting, and --fix
• add a dedicated Node 22 CI job that runs prepublishOnly before publishing

Closes #103

Validation

• npm run prepublishOnly