Move the login-shell switch behind a new modules.fish.defaultShell
option (default false; neogaia opts in). Convert every non-eza alias to
an abbreviation, set preferAbbrs and pin generateCompletions, and switch
the command line to vi-style editing. Cap navigation at four dots and
drop psmem/psmem10/dir/vdir/please. Comment the installed packages.
Translate the CachyOS fish snapshot into a home-manager Module: the
fastfetch greeting, the bat-backed manpager, the done and bang-bang
plugins (from nixpkgs, not a plugin manager), the history/backup/copy
helper functions, and the eza and navigation aliases. Pacman-specific
aliases are dropped or replaced with NixOS equivalents (update ->
nixos-rebuild switch, cleanup -> nix-collect-garbage). Enabling the
Module also makes fish the user's default login shell.
