feat: add Anthropic client wrapper

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/models/anthropic.ts

@@ -0,0 +1,46 @@
+import Anthropic from '@anthropic-ai/sdk';
+import type { ChatRequest, ChatResponse, ModelClient } from './types.js';
+
+export interface AnthropicClientConfig {
+  apiKey?: string;  // Falls back to ANTHROPIC_API_KEY env var
+  model: string;
+  maxTokens?: number;
+}
+
+export class AnthropicClient implements ModelClient {
+  private client: Anthropic;
+  private model: string;
+  private defaultMaxTokens: number;
+
+  constructor(config: AnthropicClientConfig) {
+    this.client = new Anthropic({
+      apiKey: config.apiKey,
+    });
+    this.model = config.model;
+    this.defaultMaxTokens = config.maxTokens ?? 4096;
+  }
+
+  async chat(request: ChatRequest): Promise<ChatResponse> {
+    const response = await this.client.messages.create({
+      model: this.model,
+      max_tokens: request.maxTokens ?? this.defaultMaxTokens,
+      system: request.system,
+      messages: request.messages.map((m) => ({
+        role: m.role,
+        content: m.content,
+      })),
+    });
+
+    const textContent = response.content.find((c) => c.type === 'text');
+    const content = textContent?.type === 'text' ? textContent.text : '';
+
+    return {
+      content,
+      stopReason: response.stop_reason ?? 'end_turn',
+      usage: {
+        inputTokens: response.usage.input_tokens,
+        outputTokens: response.usage.output_tokens,
+      },
+    };
+  }
+}