feat: 添加 Skills 系统支持可复用提示模板

- 新增 Skill 类型定义和加载器，支持 YAML/JSON/Markdown 格式
- 实现 Skill 注册表，支持搜索、分类和优先级覆盖
- 添加 8 个内置 Skills: code-review, explain-code, generate-docs 等
- 创建 skill 和 skill_search 工具供 Agent 调用
- 支持从用户目录和项目目录加载自定义 Skills
- 添加完整的单元测试覆盖

package-lock.json

@@ -23,6 +23,7 @@
         "vscode-jsonrpc": "^8.2.1",
         "vscode-languageserver-protocol": "^3.17.5",
         "web-tree-sitter": "^0.25.10",
+        "yaml": "^2.8.2",
         "zod": "^4.1.13"
       },
       "bin": {
@@ -3661,6 +3662,21 @@
         "node": ">=8"
       }
     },
+    "node_modules/yaml": {
+      "version": "2.8.2",
+      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.8.2.tgz",
+      "integrity": "sha512-mplynKqc1C2hTVYxd0PU2xQAc22TI1vShAYGksCCfxbn/dFwnHTNi1bvYsBTkhdUNtGIf5xNOg938rrSSYvS9A==",
+      "license": "ISC",
+      "bin": {
+        "yaml": "bin.mjs"
+      },
+      "engines": {
+        "node": ">= 14.6"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/eemeli"
+      }
+    },
     "node_modules/yoctocolors-cjs": {
       "version": "2.1.3",
       "resolved": "https://registry.npmjs.org/yoctocolors-cjs/-/yoctocolors-cjs-2.1.3.tgz",