feat: integrate skills.sh search for discovering skills from public registry

Add skills.sh API integration allowing users to search and install from
a catalog of 91K+ agent skills directly within CC Switch. The search
results are converted to DiscoverableSkill objects and reuse the existing
install pipeline. Includes fallback directory search for repos where
skills are nested in subdirectories, and filters out non-GitHub sources.

src-tauri/src/commands/skill.rs

@@ -9,6 +9,7 @@ use crate::error::format_skill_error;
 use crate::services::skill::{
     DiscoverableSkill, ImportSkillSelection, MigrationResult, Skill, SkillBackupEntry, SkillRepo,
     SkillService, SkillStorageLocation, SkillUninstallResult, SkillUpdateInfo,
+    SkillsShSearchResult,
 };
 use crate::store::AppState;
 use std::sync::Arc;
@@ -170,6 +171,18 @@ pub async fn migrate_skill_storage(
     SkillService::migrate_storage(&app_state.db, target).map_err(|e| e.to_string())
 }
 
+/// 搜索 skills.sh 公共目录
+#[tauri::command]
+pub async fn search_skills_sh(
+    query: String,
+    limit: usize,
+    offset: usize,
+) -> Result<SkillsShSearchResult, String> {
+    SkillService::search_skills_sh(&query, limit, offset)
+        .await
+        .map_err(|e| e.to_string())
+}
+
 // ========== 兼容旧 API 的命令 ==========
 
 /// 获取技能列表（兼容旧 API）