feat(runtime): DeerFlow 模式中间件链 Phase 1-4 全部完成

借鉴 DeerFlow 架构，实现完整中间件链系统：

Phase 1 - Agent 中间件链基础设施
- MiddlewareChain Clone 支持
- LoopRunner 双路径集成 (middleware/legacy)
- Kernel create_middleware_chain() 工厂方法

Phase 2 - 技能按需注入
- SkillIndexMiddleware (priority 200)
- SkillLoadTool 工具
- SkillDetail/SkillIndexEntry 结构体
- KernelSkillExecutor trait 扩展

Phase 3 - Guardrail 安全护栏
- GuardrailMiddleware (priority 400, fail_open)
- ShellExecRule / FileWriteRule / WebFetchRule

Phase 4 - 记忆闭环统一
- MemoryMiddleware (priority 150, 30s 防抖)
- after_completion 双路径调用

中间件注册顺序：
100 Compaction | 150 Memory | 200 SkillIndex
400 Guardrail  | 500 LoopGuard | 700 TokenCalibration

向后兼容：Option<MiddlewareChain> 默认 None 走旧路径

crates/zclaw-skills/src/registry.rs

@@ -133,6 +133,14 @@ impl SkillRegistry {
         manifests.values().cloned().collect()
     }
 
+    /// Synchronous snapshot of all manifests.
+    /// Uses `try_read` — returns empty map if write lock is held (should be rare at steady state).
+    pub fn manifests_snapshot(&self) -> HashMap<SkillId, SkillManifest> {
+        self.manifests.try_read()
+            .map(|guard| guard.clone())
+            .unwrap_or_default()
+    }
+
     /// Execute a skill
     pub async fn execute(
         &self,