Add fish completion support

[akira7095]

Could you please add support for fish completion?

see also: #260

[hiSandog]

If the CLI already has a central command/flag definition, generating Fish completions from that source would prevent them drifting from Bash/Zsh as options change. The completion should distinguish subcommands, enum values, filesystem paths, and options that require a protector/policy argument rather than only listing flags. A small CI smoke test using fish -n plus a few complete -C queries would catch syntax and discoverability regressions.