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-tauri/src/lib.rs

@@ -965,6 +965,7 @@ pub fn run() {
             commands::discover_available_skills,
             commands::check_skill_updates,
             commands::update_skill,
+            commands::migrate_skill_storage,
             // Skill management (legacy API compatibility)
             commands::get_skills,
             commands::get_skills_for_app,