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/builtin.rs

@@ -5,6 +5,7 @@ mod file_write;
 mod shell_exec;
 mod web_fetch;
 mod execute_skill;
+mod skill_load;
 mod path_validator;
 
 pub use file_read::FileReadTool;
@@ -12,6 +13,7 @@ pub use file_write::FileWriteTool;
 pub use shell_exec::ShellExecTool;
 pub use web_fetch::WebFetchTool;
 pub use execute_skill::ExecuteSkillTool;
+pub use skill_load::SkillLoadTool;
 pub use path_validator::{PathValidator, PathValidatorConfig};
 
 use crate::tool::ToolRegistry;
@@ -23,4 +25,5 @@ pub fn register_builtin_tools(registry: &mut ToolRegistry) {
     registry.register(Box::new(ShellExecTool::new()));
     registry.register(Box::new(WebFetchTool::new()));
     registry.register(Box::new(ExecuteSkillTool::new()));
+    registry.register(Box::new(SkillLoadTool::new()));
 }