Add connect_url to BrowserConfig so agents can connect to an external chromedp/headless-shell container via CDP instead of launching a local Chrome process.

• Workers share the container (one tab each);
• Browser Cntainer is managed externally, e.g. Docker Compose.

Config (src/config.rs)

• Added connect_url: Option to BrowserConfig
• handle_launch() branches: Browser::connect() for external, Browser::launch() for local
• handle_close() drops the struct without sending CDP Browser.close for external browsers (sending it would kill the shared sandbox and all other workers' tabs)

Docs (docs/docker.md)

• Added "External Browser" section with compose examples: host+Docker, both-in-Docker, per-agent dedicated sandboxes