refactor(fish): read the non-trivial snippets from files

Move the interactiveShellInit block and the copy function body into
modules/fish/*.fish and pull them in with builtins.readFile, so the
non-trivial fish lives in real fish files (highlighting, fish_indent)
instead of inline Nix strings. The Auto-loader only collects .nix, so
the new .fish files are ignored by it. Rendered config is unchanged.

modules/fish.nix

@@ -111,44 +111,11 @@ in
 
           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
-            '';
+            body = builtins.readFile ./fish/copy.fish;
           };
         };
 
-        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
-
-          # 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
-        '';
+        interactiveShellInit = builtins.readFile ./fish/interactive-init.fish;
       };
     };
   };