fix: include slug from frontmatter in extractMetadata function

- extractMetadata was not including the slug field from frontMatter
- Added slug: frontMatter.slug to the returned metadata object
- Now the slug check on line 144 will actually find the frontmatter slug
- Fixes glossary translation slugs (glossary-de, glossary-fr)

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

scripts/migrate-documents.js

@@ -117,7 +117,8 @@ function extractMetadata(filename, content, frontMatter) {
     version,
     author,
     tags,
-    status: 'published'
+    status: 'published',
+    slug: frontMatter.slug // Include slug from frontmatter if present
   };
 }