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-runtime/src/tool.rs

@@ -37,6 +37,39 @@ pub trait SkillExecutor: Send + Sync {
         session_id: &str,
         input: Value,
     ) -> Result<Value>;
+
+    /// Return metadata for on-demand skill loading.
+    /// Default returns `None` (skill detail not available).
+    fn get_skill_detail(&self, skill_id: &str) -> Option<SkillDetail> {
+        let _ = skill_id;
+        None
+    }
+
+    /// Return lightweight index of all available skills.
+    /// Default returns empty (no index available).
+    fn list_skill_index(&self) -> Vec<SkillIndexEntry> {
+        Vec::new()
+    }
+}
+
+/// Lightweight skill index entry for system prompt injection.
+#[derive(Debug, Clone, serde::Serialize)]
+pub struct SkillIndexEntry {
+    pub id: String,
+    pub description: String,
+    pub triggers: Vec<String>,
+}
+
+/// Full skill detail returned by `skill_load` tool.
+#[derive(Debug, Clone, serde::Serialize)]
+pub struct SkillDetail {
+    pub id: String,
+    pub name: String,
+    pub description: String,
+    pub category: Option<String>,
+    pub input_schema: Option<Value>,
+    pub triggers: Vec<String>,
+    pub capabilities: Vec<String>,
 }
 
 /// Context provided to tool execution