Remove root devshell - templates are self-contained

The root flake only defines templates. Each template (templates/vanilla/
and templates/nextjs/) contains its own complete Nix configuration with
devshell, so root-level development environment is unnecessary.

nix/modules/devshell.nix

@@ -1,45 +0,0 @@
-{inputs, lib, ...}: let
-  # FHS compatibility for NixOS (steam-run wrapper)
-  # Enable if bun-installed binaries fail to find system libraries
-  fhs = false;
-in {
-  perSystem = {system, ...}: let
-    pkgs = import inputs.nixpkgs {
-      inherit system;
-      config.allowUnfree = fhs;
-    };
-
-    fhsPackages = lib.optionals fhs [
-      pkgs.steam-run
-    ];
-
-    fhsSetup = lib.optionalString fhs ''
-      alias bun="steam-run bun"
-    '';
-  in {
-    devShells.default = pkgs.mkShell {
-      nativeBuildInputs = with pkgs;
-        [
-          bun
-          nodejs
-          typescript
-          eslint
-          just
-          prettierd
-        ]
-        ++ fhsPackages;
-
-      shellHook = ''
-        ${fhsSetup}
-
-        if [ -f package.json ]; then
-          echo "Installing dependencies..."
-          bun install
-        fi
-
-        echo ""
-        echo "💡 Run 'just' to see available commands"
-      '';
-    };
-  };
-}