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()));
 }

crates/zclaw-runtime/src/tool/builtin/skill_load.rs

@@ -0,0 +1,81 @@
+//! Skill load tool — on-demand retrieval of full skill details.
+//!
+//! When the `SkillIndexMiddleware` is active, the system prompt contains only a lightweight
+//! skill index. This tool allows the LLM to load full skill details (description, input schema,
+//! capabilities) on demand, exactly when the LLM decides a particular skill is relevant.
+
+use async_trait::async_trait;
+use serde_json::{json, Value};
+use zclaw_types::{Result, ZclawError};
+
+use crate::tool::{Tool, ToolContext};
+
+pub struct SkillLoadTool;
+
+impl SkillLoadTool {
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+#[async_trait]
+impl Tool for SkillLoadTool {
+    fn name(&self) -> &str {
+        "skill_load"
+    }
+
+    fn description(&self) -> &str {
+        "Load full details for a skill by its ID. Use this when you need to understand a skill's \
+         input parameters, capabilities, or usage instructions before calling execute_skill. \
+         Returns the skill description, input schema, and trigger conditions."
+    }
+
+    fn input_schema(&self) -> Value {
+        json!({
+            "type": "object",
+            "properties": {
+                "skill_id": {
+                    "type": "string",
+                    "description": "The ID of the skill to load details for"
+                }
+            },
+            "required": ["skill_id"]
+        })
+    }
+
+    async fn execute(&self, input: Value, context: &ToolContext) -> Result<Value> {
+        let skill_id = input["skill_id"].as_str()
+            .ok_or_else(|| ZclawError::InvalidInput("Missing 'skill_id' parameter".into()))?;
+
+        let executor = context.skill_executor.as_ref()
+            .ok_or_else(|| ZclawError::ToolError("Skill executor not available".into()))?;
+
+        match executor.get_skill_detail(skill_id) {
+            Some(detail) => {
+                let mut result = json!({
+                    "id": detail.id,
+                    "name": detail.name,
+                    "description": detail.description,
+                    "triggers": detail.triggers,
+                });
+                if let Some(schema) = &detail.input_schema {
+                    result["input_schema"] = schema.clone();
+                }
+                if let Some(cat) = &detail.category {
+                    result["category"] = json!(cat);
+                }
+                if !detail.capabilities.is_empty() {
+                    result["capabilities"] = json!(detail.capabilities);
+                }
+                Ok(result)
+            }
+            None => Err(ZclawError::ToolError(format!("Skill not found: {}", skill_id))),
+        }
+    }
+}
+
+impl Default for SkillLoadTool {
+    fn default() -> Self {
+        Self::new()
+    }
+}