feat: add native agent with conversation history

src/backends/index.ts

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+export { NativeAgent, type NativeAgentConfig } from './native/index.js';

src/backends/native/agent.test.ts

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+import { describe, it, expect, vi } from 'vitest';
+import { NativeAgent } from './agent.js';
+import type { ModelClient, ChatResponse } from '../../models/types.js';
+
+describe('NativeAgent', () => {
+  it('processes message and returns response', async () => {
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockResolvedValue({
+        content: 'Hello! How can I help you?',
+        stopReason: 'end_turn',
+        usage: { inputTokens: 10, outputTokens: 8 },
+      } satisfies ChatResponse),
+    };
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'You are Flynn, a helpful assistant.',
+    });
+
+    const response = await agent.process('Hello');
+
+    expect(response).toBe('Hello! How can I help you?');
+    expect(mockClient.chat).toHaveBeenCalledWith({
+      messages: [{ role: 'user', content: 'Hello' }],
+      system: 'You are Flynn, a helpful assistant.',
+    });
+  });
+
+  it('maintains conversation history', async () => {
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockResolvedValue({
+        content: 'Response',
+        stopReason: 'end_turn',
+        usage: { inputTokens: 10, outputTokens: 5 },
+      } satisfies ChatResponse),
+    };
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'System',
+    });
+
+    await agent.process('First message');
+    await agent.process('Second message');
+
+    expect(mockClient.chat).toHaveBeenLastCalledWith({
+      messages: [
+        { role: 'user', content: 'First message' },
+        { role: 'assistant', content: 'Response' },
+        { role: 'user', content: 'Second message' },
+      ],
+      system: 'System',
+    });
+  });
+
+  it('resets conversation history', async () => {
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockResolvedValue({
+        content: 'Response',
+        stopReason: 'end_turn',
+        usage: { inputTokens: 10, outputTokens: 5 },
+      } satisfies ChatResponse),
+    };
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'System',
+    });
+
+    await agent.process('Message 1');
+    agent.reset();
+    await agent.process('Message 2');
+
+    expect(mockClient.chat).toHaveBeenLastCalledWith({
+      messages: [{ role: 'user', content: 'Message 2' }],
+      system: 'System',
+    });
+  });
+});

src/backends/native/agent.ts

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+import type { ModelClient, Message } from '../../models/types.js';
+
+export interface NativeAgentConfig {
+  modelClient: ModelClient;
+  systemPrompt: string;
+}
+
+export class NativeAgent {
+  private modelClient: ModelClient;
+  private systemPrompt: string;
+  private history: Message[] = [];
+
+  constructor(config: NativeAgentConfig) {
+    this.modelClient = config.modelClient;
+    this.systemPrompt = config.systemPrompt;
+  }
+
+  async process(userMessage: string): Promise<string> {
+    this.history.push({ role: 'user', content: userMessage });
+
+    const response = await this.modelClient.chat({
+      messages: [...this.history],
+      system: this.systemPrompt,
+    });
+
+    this.history.push({ role: 'assistant', content: response.content });
+
+    return response.content;
+  }
+
+  reset(): void {
+    this.history = [];
+  }
+
+  getHistory(): Message[] {
+    return [...this.history];
+  }
+}

src/backends/native/index.ts

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+export { NativeAgent, type NativeAgentConfig } from './agent.js';