docs: fix README library example and add examples section

[ntegrals]

Summary

Fixes incorrect code in the README and adds missing sections.

Fixes

Library usage example was broken — used createViewport() and createModel() which don't exist in the API. Replaced with the actual working pattern:

// Before (broken)
import { Agent, createViewport, createModel } from 'open-browser'
const viewport = await createViewport({ headless: true })
const model = createModel('openai', 'gpt-4o')

// After (correct)
import { createAnthropic } from '@ai-sdk/anthropic';
import { Agent, Viewport, VercelModelAdapter } from 'open-browser';
const model = new VercelModelAdapter({ model: anthropic('claude-haiku-4-5-20251001') });
const browser = new Viewport({ headless: true });
await browser.start();

CLI default model was wrong — said gpt-4o but the actual default in run.ts is claude-haiku-4-5-20251001.

Additions

• Examples section — links to the examples/ directory with usage instructions for basic agent, multi-provider, and visible browser mode
• ARCHITECTURE.md link — added reference in the Development section

Test plan

• Library example matches actual API (verified against AgentOptions interface and Viewport class)
• CLI default model matches run.ts line 71
• Build passes