This PR adds a `debug landlock` subcommand to the Codex CLI for testing
how Codex would execute a command using the specified sandbox policy.
Built and ran this code in the `rust:latest` Docker container. In the
container, hitting the network with vanilla `curl` succeeds:
```
$ curl google.com
<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<TITLE>301 Moved</TITLE></HEAD><BODY>
<H1>301 Moved</H1>
The document has moved
<A HREF="http://www.google.com/">here</A>.
</BODY></HTML>
```
whereas this fails, as expected:
```
$ cargo run -- debug landlock -s network-restricted -- curl google.com
curl: (6) getaddrinfo() thread failed to start
```
e79549f039