feat(skills): flag unmanaged skills with a green dot on the Import button

- Add an optional `enabled` flag to useScanUnmanagedSkills; the Skills panel
  scans once on mount (enabled:true), with 30s staleTime + keepPreviousData to
  dedupe disk IO across navigations.
- App subscribes to the shared query (enabled:false) and renders a green dot +
  tooltip on the top-bar Import button when unmanaged skills are available.

src/hooks/useSkills.ts

@@ -187,12 +187,19 @@ export function useToggleSkillApp() {
 
 /**
  * 扫描未管理的 Skills
+ *
+ * - 传 { enabled: true }（Skill 面板挂载时）会在进入页面时自动静默扫描一次，
+ *   30s 内复用结果，避免来回切页时重复磁盘 IO。
+ * - 默认 enabled: false：仅订阅共享缓存（如顶栏「导入」按钮的绿点提示），
+ *   不主动触发扫描。两者共用同一 queryKey，面板扫描完成后绿点会自动亮起。
  */
-export function useScanUnmanagedSkills() {
+export function useScanUnmanagedSkills(options?: { enabled?: boolean }) {
   return useQuery({
     queryKey: ["skills", "unmanaged"],
     queryFn: () => skillsApi.scanUnmanaged(),
-    enabled: false, // 手动触发
+    enabled: options?.enabled ?? false,
+    staleTime: 30 * 1000,
+    placeholderData: keepPreviousData,
   });
 }