feat: Shell completion command

[seroperson]

This PR introduces the completion command for enabling shell completion. With click it's available out-of-box, just needed a handy command to use it easily. Tools like gh, pip, kubectl etc have such completion command too, so it's a well-known adopted pattern.

eval "$(alw completion bash)" # bash
eval "$(alw completion zsh)"  # zsh
alw completion fish | source  # fish

Recording.2026-04-15.120439.mp4

This aligns with the same functionality for gitt: entrius/gittensor#431

The only thing which differs from gitt implementation is that alw uses heavy bittensor imports, which slow completions just a little, so I had to add a stub if we're inside of completion invocation. It's a common click limitation and it has several possible workarounds, but the most suitable is just stubbing.