refactor(fish): lay the Module out like ~/.config/fish

Move the Module to modules/fish/fish.nix and mirror a real fish config
directory beside it: config.fish (interactive init), conf.d/done.fish
(the done plugin tuning, now a real conf.d file), and functions/copy.fish.
completions/ and themes/ are omitted as they have no content and git
cannot track empty directories.

modules/fish/fish.nix

@@ -0,0 +1,125 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# fish for the primary user, configured natively through home-manager. Wires the
+# done and bang-bang plugins, a fastfetch greeting, a bat-backed manpager, helper
+# functions, the eza aliases, and vi-style command-line editing. Set fish as the
+# 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";
+
+    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.${user}.shell = lib.mkIf cfg.defaultShell pkgs.fish;
+
+    home-manager.users.${user} = {
+      home.packages = with pkgs; [
+        eza # modern ls with git awareness and icons; backs the ls aliases
+        bat # syntax-highlighting cat/pager; backs the manpager below
+        fastfetch # system-info banner printed as the shell greeting
+        wget # non-interactive HTTP downloader; backs the wget abbreviation
+      ];
+
+      # done's tuning lives in its own conf.d snippet, mirroring ~/.config/fish.
+      xdg.configFile."fish/conf.d/done.fish".source = ./conf.d/done.fish;
+
+      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 = [
+          {
+            name = "done";
+            src = pkgs.fishPlugins.done.src;
+          }
+          {
+            name = "bang-bang";
+            src = pkgs.fishPlugins.bang-bang.src;
+          }
+        ];
+
+        # Only the eza listings stay aliases; everything else is an abbreviation.
+        shellAliases = {
+          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 ../../..";
+
+          vi = "nvim";
+          vim = "nvim";
+          cp = "cp -v";
+          tmx = "tmux new-session -A -s";
+
+          tarnow = "tar -acf ";
+          untar = "tar -zxvf ";
+          wget = "wget -c ";
+          grep = "grep --color=auto";
+          fgrep = "fgrep --color=auto";
+          egrep = "egrep --color=auto";
+          jctl = "journalctl -p 3 -xb";
+
+          # 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 = builtins.readFile ./functions/copy.fish;
+          };
+        };
+
+        interactiveShellInit = builtins.readFile ./config.fish;
+      };
+    };
+  };
+}