refactor: 创建 @anthropic-ai/model-provider 包骨架与类型定义

- 新建 workspace 包 packages/@anthropic-ai/model-provider
- 定义 ModelProviderHooks 接口（依赖注入：分析、成本、日志等）
- 定义 ClientFactories 接口（Anthropic/OpenAI/Gemini/Grok 客户端工厂）
- 搬入核心类型：Message 体系、NonNullableUsage、EMPTY_USAGE、SystemPrompt、错误常量
- 主项目 src/types/message.ts 等改为 re-export，保持向后兼容

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/@anthropic-ai/model-provider/package.json

@@ -0,0 +1,18 @@
+{
+  "name": "@anthropic-ai/model-provider",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "exports": {
+    ".": "./src/index.ts",
+    "./types": "./src/types/index.ts",
+    "./hooks": "./src/hooks/index.ts",
+    "./client": "./src/client/index.ts"
+  },
+  "dependencies": {
+    "@anthropic-ai/sdk": "^0.80.0",
+    "openai": "^6.33.0"
+  }
+}