refactor: 删除外置 skills 系统，全部工具转为内置

删除:
- resources/skills/ (9个外置 skill, bash/python/node 脚本)
- src/tools/registry.rs (SkillRegistry 加载/执行)
- src/tools/parser.rs (SKILL.md 解析)
- src/skills_importer.rs (skills 导入)
- cli.rs Skills 子命令
- main.rs cmd_skills / global_skills_dir

保留:
- src/tools/builtin.rs (内置工具注册 + 审批流程)
- src/tools/weather.rs (天气查询)
- src/tools/skill.rs (精简为 SkillSpec/RiskLevel/SkillResult/ExecutionContext)
- src/tools/approval.rs (审批管理器)

main.rs 简化: 移除 SkillRegistry 加载, executor 只走 BuiltinRegistry

src/tools/approval.rs

@@ -44,7 +44,7 @@ impl ApprovalManager {
         user_id: &str,
         skill_name: &str,
     ) -> Result<(String, oneshot::Receiver<ApprovalDecision>), String> {
-        let code: u32 = rand::thread_rng().gen_range(100000..999999);
+        let code: u32 = rand::rng().random_range(100000..999999);
         let code_str = code.to_string();
         let code_hash = format!("{:x}", Sha256::digest(code_str.as_bytes()));
 
@@ -92,7 +92,9 @@ impl ApprovalManager {
         let (result, name, code_hash) = {
             let mut map = self.pending.lock().await;
             let entries = map.get_mut(user_id)?;
-            if entries.is_empty() { return None; }
+            if entries.is_empty() {
+                return None;
+            }
 
             let reply_hash = format!("{:x}", Sha256::digest(reply.as_bytes()));
             let reply_trimmed = reply.trim();
@@ -157,11 +159,11 @@ impl ApprovalManager {
         }
 
         if let Some(ref pool) = self.pool {
-            for (_uid, _i, _code_hash) in &expired {
+            for (_uid, _i, code_hash) in &expired {
                 let _ = sqlx::query(
                     "UPDATE pending_approvals SET status = 'expired' WHERE code_hash = $1 AND status = 'pending'",
                 )
-                .bind(_code_hash)
+                .bind(code_hash)
                 .execute(pool.as_ref())
                 .await;
             }
@@ -178,10 +180,4 @@ impl ApprovalManager {
         }
         map.retain(|_, v| !v.is_empty());
     }
-
-    /// 获取某个用户的待审批条数
-    pub async fn pending_count(&self, user_id: &str) -> usize {
-        let map = self.pending.lock().await;
-        map.get(user_id).map(|v| v.len()).unwrap_or(0)
-    }
 }