refactor(fish): split config.fish into fragments concatenated by Nix

Break the interactive init into concern-scoped fish files under
modules/fish/config/ (bindings, env, done, path) and assemble them with
lib.concatMapStringsSep + readFile into a single interactiveShellInit, so
home-manager still writes one ~/.config/fish/config.fish. The pieces stay
editable as separate fish files; the rendered file is unchanged.

modules/fish/fish.nix

@@ -115,7 +115,15 @@ in
           };
         };
 
-        interactiveShellInit = builtins.readFile ./config.fish;
+        # 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
+        ];
       };
     };
   };