fix: normalize bare-string tags to array in parseFrontmatter

When frontmatter contains tags: single-value without bracket syntax,
the generic parser assigned a plain string to result.tags. Downstream
consumers iterate with for...of which on a string yields individual
characters, producing garbled single-character tag categories in catalog
and corrupted BM25 search index data. Normalize to a single-element
array after parsing.

Author: cursoragent

workers/src/zip-baseline-fetcher.ts

@@ -132,6 +132,10 @@ function parseFrontmatter(content: string): FrontmatterResult {
     result[key] = value;
   }
 
+  if (typeof result.tags === "string") {
+    result.tags = [result.tags];
+  }
+
   return result;
 }