Add a 'hivectl completions <shell>' subcommand (clap_complete) that
prints a completion script for bash/zsh/fish/elvish/powershell, generated
from hivectl's own clap command tree so it never drifts from the real
verbs/flags. The package build installs the bash/zsh/fish scripts via
installShellFiles, so an operator gets working completion automatically
once hivectl is on their profile with shell completion enabled.
Regenerated docs/tools/hivectl-cli.md for the new verb.
c7612dcf2b