Add zmx completion script

[ldsands]

Adds nushell completions for zmx, a terminal session manager (attach/detach to persistent shell sessions).

The new custom-completions/zmx/zmx-completions.nu covers:

• All zmx subcommands and their short aliases (a, r, s, p, wr, d, l/ls, k, hi, w, t, c, v, h)
• Per-command flags (run -d --fish, list --short, kill --force, history --vt --html)
• Dynamic session-name completion for attach, run, send, print, write, kill, history, wait, and tail via zmx list --short
• A top-level zmx extern so zmx <TAB> lists subcommands with descriptions
• Shell-name completion for zmx completions <TAB> (bash/zsh/fish/nu)

A short README.md is included following the custom-completions/ convention.

[ldsands]

Full disclosure I did this all on claude but I tested it and it seems to work very well.

Thanks in advance for all your work!

[fdncred]

Thanks