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/client/index.ts

@@ -0,0 +1,27 @@
+import type { ClientFactories } from './types.js'
+
+let registeredFactories: ClientFactories | null = null
+
+/**
+ * Register client factories from the main project.
+ * Call this during application initialization.
+ */
+export function registerClientFactories(factories: ClientFactories): void {
+  registeredFactories = factories
+}
+
+/**
+ * Get registered client factories.
+ * Throws if not registered (fail-fast).
+ */
+export function getClientFactories(): ClientFactories {
+  if (!registeredFactories) {
+    throw new Error(
+      'Client factories not registered. ' +
+        'Call registerClientFactories() during app initialization.',
+    )
+  }
+  return registeredFactories
+}
+
+export type { ClientFactories }

packages/@anthropic-ai/model-provider/src/client/types.ts

@@ -0,0 +1,35 @@
+/**
+ * Client factory interfaces.
+ * Authentication is handled externally — main project provides factory implementations.
+ */
+export interface ClientFactories {
+  /** Get Anthropic client (1st party, Bedrock, Foundry, Vertex) */
+  getAnthropicClient: (params: {
+    model?: string
+    maxRetries: number
+    fetchOverride?: unknown
+    source?: string
+  }) => Promise<unknown>
+
+  /** Get OpenAI-compatible client */
+  getOpenAIClient: (params: {
+    maxRetries: number
+    fetchOverride?: unknown
+    source?: string
+  }) => unknown
+
+  /** Stream Gemini generate content */
+  streamGeminiGenerateContent: (params: {
+    model: string
+    signal?: AbortSignal
+    fetchOverride?: unknown
+    body: Record<string, unknown>
+  }) => AsyncIterable<unknown>
+
+  /** Get Grok client (OpenAI-compatible) */
+  getGrokClient: (params: {
+    maxRetries: number
+    fetchOverride?: unknown
+    source?: string
+  }) => unknown
+}