Trivial sandbox escape

In unit6, [Tools and Sandboxing](https://huggingface.co/learn/context-course/en/unit6/tools-and-sandboxing) the harness sandbox has a trivial escape.

The [rg command with the --pre flag](https://man.archlinux.org/man/extra/ripgrep/rg.1.en#:~:text=%2D%2Dpre%3DCOMMAND) allows you to run arbitrary commands.

This section would be better served with a discussion of actual sandbox tools like bubblewrap, firejail, the underlying security features like seccomp and landlock, and microvms like firecracker.

Start with the linked unit6 Tools and Sandboxing section and review how the harness uses ripgrep with the --pre flag. Update the section to address the escape and cover the requested sandbox tools, security features, and microVMs; done means the guidance no longer presents an escapable setup without warning.

Written by the indexing model from the issue text.