feat(nix): add nixfmt-rfc-style to dev dependencies and lefthook

Add nixfmt-rfc-style as a development dependency in the Nix flake and
configure lefthook to automatically format .nix files on pre-commit.

Uses `nix develop --command` to run nixfmt from the devShell context,
ensuring consistent Nix code formatting across the project.

Author: ryoppippi

flake.nix

@@ -6,30 +6,34 @@
     flake-parts.url = "github:hercules-ci/flake-parts";
   };
 
-  outputs = inputs@{ flake-parts, ... }:
+  outputs =
+    inputs@{ flake-parts, ... }:
     flake-parts.lib.mkFlake { inherit inputs; } {
       systems = [
         "x86_64-linux"
         "aarch64-linux"
         "aarch64-darwin"
       ];
 
-      perSystem = { pkgs, ... }: {
-        devShells.default = pkgs.mkShell {
-          buildInputs = with pkgs; [
-            pnpm_10
-          ];
+      perSystem =
+        { pkgs, ... }:
+        {
+          devShells.default = pkgs.mkShell {
+            buildInputs = with pkgs; [
+              pnpm_10
+              nixfmt-rfc-style
+            ];
 
-          shellHook = ''
-            echo "StackOne AI Node SDK development environment"
+            shellHook = ''
+              echo "StackOne AI Node SDK development environment"
 
-            # Install dependencies only if node_modules/.pnpm/lock.yaml is older than pnpm-lock.yaml
-            if [ ! -f node_modules/.pnpm/lock.yaml ] || [ pnpm-lock.yaml -nt node_modules/.pnpm/lock.yaml ]; then
-              echo "📦 Installing dependencies..."
-              pnpm install --frozen-lockfile
-            fi
-          '';
+              # Install dependencies only if node_modules/.pnpm/lock.yaml is older than pnpm-lock.yaml
+              if [ ! -f node_modules/.pnpm/lock.yaml ] || [ pnpm-lock.yaml -nt node_modules/.pnpm/lock.yaml ]; then
+                echo "📦 Installing dependencies..."
+                pnpm install --frozen-lockfile
+              fi
+            '';
+          };
         };
-      };
     };
 }

lefthook.yaml

@@ -9,6 +9,10 @@ pre-commit:
       glob: '*'
       run: pnpm oxfmt --no-error-on-unmatched-pattern {staged_files}
       stage_fixed: true
+    - name: nixfmt
+      glob: '*.nix'
+      run: nix develop --command nixfmt {staged_files}
+      stage_fixed: true
 
 pre-push:
   jobs: