fix: searchSkills 使用缓存 IDF 前校验 index 引用一致性，修复测试间歇性失败

Author: claude-code-best

src/services/skillSearch/localSearch.ts

@@ -397,7 +397,7 @@ export function searchSkills(
   for (const v of freq.values()) if (v > max) max = v
   for (const [term, count] of freq) queryTf.set(term, count / max)
 
-  const idf = cachedIdf ?? computeIdf(index)
+  const idf = cachedIndex === index && cachedIdf ? cachedIdf : computeIdf(index)
   const queryTfIdf = new Map<string, number>()
   for (const [term, tf] of queryTf) {
     queryTfIdf.set(term, tf * (idf.get(term) ?? 0))