Remove columns field to fix KeyStatic error

The columns field might be causing compatibility issues with KeyStatic 5.x.

Co-Authored-By: Claude <noreply@anthropic.com>

keystatic.config.ts

@@ -23,7 +23,6 @@ export default config({
         contentField: 'body',
         frontmatter: 'yaml',
       },
-      columns: ['title', 'date', 'author'],
       schema: {
         title: fields.slug({
           name: {