fix(chat): 定时功能审计修复 — 消除重复解析 + ID碰撞 + 输入补全

审计发现修复：
- H-01: 存储 ParsedSchedule 避免重复 parse_nl_schedule 调用
- H-03: trigger ID 追加 UUID 片段防止高并发碰撞
- C-02: execute_trigger 验证错误信息明确系统 Hand 必须注册
- M-02: SchedulerService 传递 trigger_name 作为 task_description
- M-01: 添加拦截路径跳过 post_hook 的设计注释

crates/zclaw-kernel/src/trigger_manager.rs

@@ -305,9 +305,10 @@ impl TriggerManager {
         };
 
         // Get hand (outside of our lock to avoid potential deadlock with hand_registry)
+        // System hands (prefixed with '_') must be registered at boot — same rule as create_trigger.
         let hand = self.hand_registry.get(&hand_id).await
             .ok_or_else(|| zclaw_types::ZclawError::InvalidInput(
-                format!("Hand '{}' not found", hand_id)
+                format!("Hand '{}' not found (system hands must be registered at boot)", hand_id)
             ))?;
 
         // Update state before execution