docs: add sandbox examples for WarpGrep remoteCommands

Add 6 platform-specific examples showing how to use WarpGrep
with remoteCommands in cloud sandboxes:

- E2B Sandbox
- Modal Sandbox
- Daytona Sandbox
- Vercel Sandbox
- Cloudflare Worker Sandbox (pattern reference)
- Docker/SSH

Each example is self-contained with package.json, agent.ts,
and README.md. All use the Anthropic agent loop pattern.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>