feat: add lerd cpx

[geodro]

cpx is to Composer what npx is to npm, a way to run a command from any Composer package without adding it to the project. Routing it through lerd is what makes it run on the PHP version the site is registered on rather than whatever PHP the host happens to have, the same reason lerd wraps composer and artisan.

Worth saying plainly: most of this already worked before the command existed. Requiring cpx globally makes lerd sync a shim onto PATH like it does for any composer global binary, and lerd already execs with HOME set to the host's home, so the packages cpx fetches cache under ~/.cpx and survive a container restart. What the command adds is the explicit form and an error worth reading, since without it a missing cpx fails inside the container with "could not open input file" rather than naming the one command that fixes it.

The binary is resolved through the same COMPOSER_HOME and XDG lookup the global bin sync uses, so a non-default composer home is followed rather than guessed at.

Confirmed against a real project on 8.5, running a package the project does not have:

$ lerd cpx phpstan/phpstan analyse app --level=1
{"tool":"phpstan","result":"passed","errors":0}

Note the package is cpx/cpx, not laravel/cpx as the repo name suggests.

Closes #1543