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/src/hooks/index.ts

@@ -0,0 +1,27 @@
+import type { ModelProviderHooks } from './types.js'
+
+let registeredHooks: ModelProviderHooks | null = null
+
+/**
+ * Register hooks from the main project.
+ * Call this during application initialization.
+ */
+export function registerHooks(hooks: ModelProviderHooks): void {
+  registeredHooks = hooks
+}
+
+/**
+ * Get registered hooks.
+ * Throws if hooks not registered (fail-fast).
+ */
+export function getHooks(): ModelProviderHooks {
+  if (!registeredHooks) {
+    throw new Error(
+      'ModelProvider hooks not registered. ' +
+        'Call registerHooks() during app initialization.',
+    )
+  }
+  return registeredHooks
+}
+
+export type { ModelProviderHooks }