refactor(fish): merge the config fragments back into one config.fish

The interactive init is small; four fragment files was over-splitting it.
Keep the mechanic (a real fish file inlined by Nix into config.fish) but
merge the fragments into a single modules/fish/config.fish read with
builtins.readFile. Rendered config is unchanged.

modules/fish/fish.nix

@@ -115,15 +115,9 @@ in
           };
         };
 
-        # config.fish is assembled here, in order, from the fragments under
-        # ./config so each concern stays an editable fish file while
-        # home-manager still writes a single ~/.config/fish/config.fish.
-        interactiveShellInit = lib.concatMapStringsSep "\n" builtins.readFile [
-          ./config/bindings.fish
-          ./config/env.fish
-          ./config/done.fish
-          ./config/path.fish
-        ];
+        # Read from a real fish file, which home-manager renders into
+        # ~/.config/fish/config.fish.
+        interactiveShellInit = builtins.readFile ./config.fish;
       };
     };
   };