docs: add runnable example scripts

[ntegrals]

Summary

Adds an examples/ directory with 6 runnable scripts demonstrating core Open Browser features. Each example is self-contained, type-checked, and includes usage instructions in a doc comment.

Examples

Example	What it demonstrates
basic-agent.ts	Simplest possible usage — give a task, get a result
extract-data.ts	Extract structured data from a web page
multi-provider.ts	Switch between OpenAI, Anthropic, and Google with a CLI arg
step-callbacks.ts	Monitor agent progress with onStepStart/onStepEnd/onDone hooks
headless-vs-visible.ts	Run headless or with a visible browser window
url-security.ts	Restrict navigation with allowedUrls on Viewport

Running

# Set an API key
export ANTHROPIC_API_KEY=sk-...

# Run any example
bun run examples/basic-agent.ts
bun run examples/multi-provider.ts openai
bun run examples/headless-vs-visible.ts visible

Design choices

• All examples use claude-haiku-4-5-20251001 for low cost
• Every example includes try/finally with browser.close() for clean shutdown
• Examples import from open-browser (the workspace package), so they work immediately after bun install
• No external dependencies beyond what's already in the monorepo

Test plan

• All 6 examples pass tsc --noEmit type checking
• Manual: bun run examples/basic-agent.ts completes successfully
• Manual: bun run examples/multi-provider.ts anthropic works