next lint no longer exists as of Next.js 16.0.0, and next build does not lint your code as a side effect anymore.
The fix is a two-line eslint.config.mjs built from eslint-config-next/core-web-vitals and eslint-config-next/typescript, run through the ESLint CLI.
If you skip this migration, your build stays green while lint errors ship to production unchecked.