Fix Codex skill version metadata (#703)

Move Codex and .agents skill versions under metadata while keeping all version readers compatible with legacy top-level frontmatter.\n\nAI assistance: prepared with Codex under maintainer direction.

scripts/lib/utils.js

@@ -787,6 +787,9 @@ export function generateYamlFrontmatter(data) {
           lines.push(`  - ${formatYamlScalar(item)}`);
         }
       }
+    } else if (value && typeof value === 'object') {
+      lines.push(`${key}:`);
+      appendYamlObject(lines, value, 2);
     } else if (typeof value === 'boolean') {
       lines.push(`${key}: ${value}`);
     } else {