fix(nix): replace deprecated nixfmt-rfc-style with nixfmt

nixfmt-rfc-style is now the same as pkgs.nixfmt and should be used
instead. This removes the explicit package override in treefmt and
updates devShells to use the standard nixfmt package.

- Remove nixfmt.package override in treefmt config (defaults to nixfmt)
- Replace nixfmt-rfc-style with nixfmt in devShells.default
- Update nix-workflow.md documentation to reflect the change

Author: ryoppippi

.claude/rules/nix-workflow.md

@@ -16,7 +16,7 @@ devShells.default = pkgs.mkShellNoCC {
     uv
     ty
     just
-    nixfmt-rfc-style
+    nixfmt
 
     # your new tool here
     new-tool

flake.nix

@@ -39,7 +39,6 @@
             projectRootFile = "flake.nix";
             programs = {
               nixfmt.enable = true;
-              nixfmt.package = pkgs.nixfmt-rfc-style;
               ruff-check.enable = true;
               ruff-format.enable = true;
             };
@@ -94,7 +93,7 @@
               uv
               ty
               just
-              nixfmt-rfc-style
+              nixfmt
 
               # security
               gitleaks