fix: 优先审批 + 首用户隔离 + 审批DB闭环 + memo错误

1. (High) 内置工具先审批再执行 — is_builtin 检查优先，
   High 风险通过 builtin_approve 后再调用 BuiltinRegistry
2. (High) 首用户 current_user_id — 移除 !is_empty() 守卫
3. (Medium) 审批 DB 状态闭环 — 取消/错误时按 user_id 更新最近 pending
4. (Medium) manage_memos 写入失败 — .ok() → 完整错误返回

src/tools/builtin.rs

@@ -36,6 +36,10 @@ impl BuiltinRegistry {
     pub fn is_high_risk(name: &str) -> bool {
         Self::specs().iter().any(|s| s.name == name && s.risk_level == RiskLevel::High)
     }
+
+    pub fn is_builtin(name: &str) -> bool {
+        Self::specs().iter().any(|s| s.name == name)
+    }
 }
 
 fn execute_datetime() -> SkillResult {
@@ -64,7 +68,9 @@ async fn handle_memos(args_json: &str) -> SkillResult {
                 "id": next_id, "content": content,
                 "created_at": Local::now().format("%Y-%m-%d %H:%M").to_string()
             }));
-            std::fs::write(path, serde_json::to_string_pretty(&data).unwrap()).ok();
+            if let Err(e) = std::fs::write(path, serde_json::to_string_pretty(&data).unwrap()) {
+                return SkillResult::error(format!("写入备忘录失败: {}", e));
+            }
             SkillResult::ok(format!("已添加备忘录 #{}", next_id))
         }
         "list" => {
@@ -80,7 +86,9 @@ async fn handle_memos(args_json: &str) -> SkillResult {
             let before = data.len();
             data.retain(|m| m.get("id").and_then(|v| v.as_i64()) != Some(id));
             if data.len() == before { return SkillResult::error(format!("未找到备忘录 #{}", id)); }
-            std::fs::write(path, serde_json::to_string_pretty(&data).unwrap()).ok();
+            if let Err(e) = std::fs::write(path, serde_json::to_string_pretty(&data).unwrap()) {
+                return SkillResult::error(format!("写入备忘录失败: {}", e));
+            }
             SkillResult::ok(format!("已删除备忘录 #{}", id))
         }
         _ => SkillResult::error("未知操作，支持: add, list, delete"),