feat(fish): add a native Module and set it as neogaia's login shell

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.

modules/fish.nix

@@ -0,0 +1,141 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# fish for the primary user, configured natively through home-manager and set
+# as their default login shell. Wires the done and bang-bang plugins, a
+# fastfetch greeting, a bat-backed manpager, helper functions, and the eza and
+# navigation aliases.
+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";
+
+  config = lib.mkIf cfg.enable {
+    # System-level fish: registers it in /etc/shells and installs vendor
+    # completions.
+    programs.fish.enable = true;
+    users.users.${user}.shell = pkgs.fish;
+
+    home-manager.users.${user} = {
+      home.packages = with pkgs; [
+        eza
+        bat
+        fastfetch
+        wget
+      ];
+
+      programs.fish = {
+        enable = true;
+
+        # done notifies when a long command finishes; bang-bang restores the !!
+        # and !$ history bindings.
+        plugins = [
+          {
+            name = "done";
+            src = pkgs.fishPlugins.done.src;
+          }
+          {
+            name = "bang-bang";
+            src = pkgs.fishPlugins.bang-bang.src;
+          }
+        ];
+
+        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 '^\\.'";
+
+          # Walk up the tree.
+          ".." = "cd ..";
+          "..." = "cd ../..";
+          "...." = "cd ../../..";
+          "....." = "cd ../../../..";
+          "......" = "cd ../../../../..";
+
+          vi = "nvim";
+          vim = "nvim";
+          cp = "cp -v";
+          tmx = "tmux new-session -A -s";
+
+          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";
+          cleanup = "sudo nix-collect-garbage -d";
+        };
+
+        functions = {
+          # Run fastfetch as the welcome message.
+          fish_greeting = "fastfetch";
+
+          history = {
+            description = "Show command history with timestamps";
+            body = "builtin history --show-time='%F %T '";
+          };
+
+          backup = {
+            description = "Copy <file> to <file>.bak";
+            argumentNames = "filename";
+            body = "cp $filename $filename.bak";
+          };
+
+          copy = {
+            description = "Copy a file, or recursively copy a source directory into a destination";
+            body = ''
+              set count (count $argv | tr -d \n)
+              if test "$count" = 2; and test -d "$argv[1]"
+                  set from (echo $argv[1] | trim-right /)
+                  set to (echo $argv[2])
+                  command cp -r $from $to
+              else
+                  command cp $argv
+              end
+            '';
+          };
+        };
+
+        interactiveShellInit = ''
+          set -gx EDITOR nvim
+          set -gx VISUAL nvim
+
+          # Render man pages through bat.
+          set -x MANROFFOPT "-c"
+          set -x MANPAGER "sh -c 'col -bx | bat -l man -p'"
+
+          # Tune the done plugin: only notify for commands past 10s, at low urgency.
+          set -g __done_min_cmd_duration 10000
+          set -g __done_notification_urgency_level low
+
+          # Prepend ~/.local/bin to PATH when it exists.
+          if test -d ~/.local/bin
+              fish_add_path ~/.local/bin
+          end
+
+          # Apply fish-compatible profile overrides if present.
+          if test -f ~/.fish_profile
+              source ~/.fish_profile
+          end
+        '';
+      };
+    };
+  };
+}