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/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