feat: add skill storage location toggle between CC Switch and ~/.agents/skills

Allow users to choose between storing skills in CC Switch's managed
directory (~/.cc-switch/skills/) or the Agent Skills open standard
directory (~/.agents/skills/). Includes migration logic that safely
moves files before updating settings, with confirmation dialog for
non-empty installations.

src/lib/api/skills.ts

@@ -88,6 +88,13 @@ export interface SkillUpdateInfo {
   remoteHash: string;
 }
 
+/** 存储位置迁移结果 */
+export interface MigrationResult {
+  migratedCount: number;
+  skippedCount: number;
+  errors: string[];
+}
+
 /** 仓库配置 */
 export interface SkillRepo {
   owner: string;
@@ -169,6 +176,13 @@ export const skillsApi = {
     return await invoke("update_skill", { id });
   },
 
+  /** 迁移 Skill 存储位置 */
+  async migrateStorage(
+    target: "cc_switch" | "unified",
+  ): Promise<MigrationResult> {
+    return await invoke("migrate_skill_storage", { target });
+  },
+
   // ========== 兼容旧 API ==========
 
   /** 获取技能列表（兼容旧 API） */

src/lib/schemas/settings.ts

@@ -29,6 +29,7 @@ export const settingsSchema = z.object({
 
   // Skill 同步设置
   skillSyncMethod: z.enum(["auto", "symlink", "copy"]).optional(),
+  skillStorageLocation: z.enum(["cc_switch", "unified"]).optional(),
 
   // WebDAV v2 同步设置（通过专用命令保存，schema 仅用于读取）
   webdavSync: z