chore(commitlint): convert config to ESM for module project

The root package.json sets "type": "module", so Node treated
commitlint.config.js as ESM and threw "module is not defined" when
loading the CommonJS module.exports — the config never applied. Switch
to `export default` so the conventional filename stays valid and the
rules are actually enforced by the CLI.

commitlint.config.js

@@ -1,4 +1,4 @@
-module.exports = {
+export default {
   extends: ['@commitlint/config-conventional'],
   rules: {
     // Enforce conventional commit types