Add fx co-located agent cookbook

[dprevoznik]

Summary

• New integration cookbook showing how to run Vercel's fx coding agent inside a Kernel browser VM, co-located with the Chromium instance it drives, so tool calls hit the VM's local playwright daemon over loopback instead of going through Kernel's public API.
• Uses process execution and file I/O to upload and run the fx binary inside the session.
• Adds the cookbook to the root README's Integrations table.

Test plan

• Reviewed CLI flags (browsers create, browsers fs upload, browsers process exec) against the installed kernel CLI (v0.34.0) --help output.
• Not run end-to-end (requires a live KERNEL_API_KEY and AI_GATEWAY_API_KEY).