feat(fish): gate the default shell and prefer abbreviations

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.

.claude/tasks/0005-fish-shell-module.md

@@ -32,3 +32,10 @@ Configure the plugins natively through home-manager rather than a fish plugin ma
 - **`copy` kept verbatim** (including the upstream `trim-right` call) to preserve exact parity with the current shell.
 - **Verification.** The `neogaia` system toplevel builds (the spec's primary seam).
   The rendered `~/.config/fish/` was inspected in the build output: aliases, the three helper functions, the `fastfetch` greeting, the bat manpager, the `done` tuning vars, and the `conf.d/plugin-done.fish` + `conf.d/plugin-bang-bang.fish` plugin files are all present; `users.users.alexion.shell` resolves to `pkgs.fish`.
+
+### Post-review adjustments
+
+- **`defaultShell` option.** Setting fish as the login shell moved behind `modules.fish.defaultShell` (default `false`, gated with `mkIf`); `neogaia` opts in explicitly. Enabling the Module alone no longer changes the login shell.
+- **Abbreviation-first.** Every non-eza alias is now a `shellAbbr` (the eza `ls` family stays an alias), `preferAbbrs = true`, and `generateCompletions = true` is pinned rather than left to the upstream default.
+- **vi command-line editing.** `interactiveShellInit` sets `fish_key_bindings fish_vi_key_bindings`; the `bang-bang` plugin re-binds `!`/`$` in insert mode via its own `--on-variable fish_key_bindings` handler, so the switch keeps them working.
+- **Trimmed aliases.** Navigation capped at four dots (`.....`/`......` dropped); `psmem`, `psmem10`, `dir`, `vdir`, and `please` removed.

hosts/neogaia/default.nix

@@ -37,6 +37,7 @@
 
   # fish as the login shell.
   modules.fish.enable = true;
+  modules.fish.defaultShell = true;
 
   # Locale preferences for the base system.
   time.timeZone = "America/New_York";

modules/fish.nix

@@ -4,34 +4,50 @@
   pkgs,
   ...
 }:
-# fish for the primary user, configured natively through home-manager and set
+# fish for the primary user, configured natively through home-manager. Wires the
-# as their default login shell. Wires the done and bang-bang plugins, a
+# done and bang-bang plugins, a fastfetch greeting, a bat-backed manpager, helper
-# fastfetch greeting, a bat-backed manpager, helper functions, and the eza and
+# functions, the eza aliases, and vi-style command-line editing. Set fish as the
-# navigation aliases.
+# default login shell by also turning on `modules.fish.defaultShell`.
 let
   cfg = config.modules.fish;
   user = config.user.name;
 in
 {
-  options.modules.fish.enable = lib.mkEnableOption "fish as the user's shell, configured via home-manager";
+  options.modules.fish = {
+    enable = lib.mkEnableOption "fish as the user's shell, configured via home-manager";
+
+    defaultShell = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = "Set fish as the user's default login shell.";
+    };
+  };
 
   config = lib.mkIf cfg.enable {
     # System-level fish: registers it in /etc/shells and installs vendor
     # completions.
     programs.fish.enable = true;
-    users.users.${user}.shell = pkgs.fish;
+    users.users.${user}.shell = lib.mkIf cfg.defaultShell pkgs.fish;
 
     home-manager.users.${user} = {
       home.packages = with pkgs; [
-        eza
+        eza # modern ls with git awareness and icons; backs the ls aliases
-        bat
+        bat # syntax-highlighting cat/pager; backs the manpager below
-        fastfetch
+        fastfetch # system-info banner printed as the shell greeting
-        wget
+        wget # non-interactive HTTP downloader; backs the wget abbreviation
       ];
 
       programs.fish = {
         enable = true;
 
+        # Relied-on upstream defaults, pinned so a future change can't silently
+        # alter behaviour.
+        generateCompletions = true;
+
+        # Prefer abbreviations over aliases when other modules wire up fish
+        # shortcuts, matching the abbreviation-first style below.
+        preferAbbrs = true;
+
         # done notifies when a long command finishes; bang-bang restores the !!
         # and !$ history bindings.
         plugins = [
@@ -45,20 +61,20 @@ in
           }
         ];
 
+        # Only the eza listings stay aliases; everything else is an abbreviation.
         shellAliases = {
-          # Replace ls with eza.
           ls = "eza -al --color=always --group-directories-first --icons";
           la = "eza -a --color=always --group-directories-first --icons";
           ll = "eza -l --color=always --group-directories-first --icons";
           lt = "eza -aT --color=always --group-directories-first --icons";
           "l." = "eza -a | grep -e '^\\.'";
+        };
 
+        shellAbbrs = {
           # Walk up the tree.
           ".." = "cd ..";
           "..." = "cd ../..";
           "...." = "cd ../../..";
-          "....." = "cd ../../../..";
-          "......" = "cd ../../../../..";
 
           vi = "nvim";
           vim = "nvim";
@@ -68,15 +84,10 @@ in
           tarnow = "tar -acf ";
           untar = "tar -zxvf ";
           wget = "wget -c ";
-          psmem = "ps auxf | sort -nr -k 4";
-          psmem10 = "ps auxf | sort -nr -k 4 | head -10";
-          dir = "dir --color=auto";
-          vdir = "vdir --color=auto";
           grep = "grep --color=auto";
           fgrep = "fgrep --color=auto";
           egrep = "egrep --color=auto";
           jctl = "journalctl -p 3 -xb";
-          please = "sudo";
 
           # Rebuild the system, and reclaim disk from old generations.
           update = "sudo nixos-rebuild switch";
@@ -114,6 +125,9 @@ in
         };
 
         interactiveShellInit = ''
+          # vi-style modal editing on the command line.
+          set -g fish_key_bindings fish_vi_key_bindings
+
           set -gx EDITOR nvim
           set -gx VISUAL nvim