feat(skills): SemanticSkillRouter — TF-IDF + Embedding 混合路由

实现 SemanticSkillRouter 核心模块 (zclaw-skills):
- Embedder trait + NoOpEmbedder (TF-IDF fallback)
- SkillTfidfIndex 全文索引
- retrieve_candidates() Top-K 检索
- route() 置信度阈值路由 (0.85)
- cosine_similarity 公共函数
- 单元测试覆盖

Kernel 适配层 (zclaw-kernel):
- EmbeddingAdapter: zclaw-growth EmbeddingClient → Embedder

文档同步:
- 01-intelligent-routing.md Phase 1+2 标记完成

crates/zclaw-kernel/src/lib.rs

@@ -9,6 +9,7 @@ mod events;
 pub mod trigger_manager;
 pub mod config;
 pub mod scheduler;
+pub mod skill_router;
 #[cfg(feature = "multi-agent")]
 pub mod director;
 pub mod generation;