chore: remove flake-parts dependency (#793)

Simplify flake.nix by removing flake-parts and using plain Nix.
The project only defines a single devShell, which doesn't require
the complexity of flake-parts. Multi-system support is achieved
with a simple forAllSystems helper using nixpkgs.lib.genAttrs.

This reduces dependencies and makes the flake more straightforward.

Author: ryoppippi

flake.lock

@@ -1,22 +1,15 @@
 {
   description = "Usage analysis tool for Claude Code";
 
-  inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
-    flake-parts.url = "github:hercules-ci/flake-parts";
-  };
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
 
-  outputs = inputs@{ flake-parts, nixpkgs, ... }:
-    flake-parts.lib.mkFlake { inherit inputs; } {
-      systems = [
-        "x86_64-linux"
-        "aarch64-linux"
-        "x86_64-darwin"
-        "aarch64-darwin"
-      ];
-
-      perSystem = { pkgs, ... }: {
-        devShells.default = pkgs.mkShell {
+  outputs = { nixpkgs, ... }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+    in {
+      devShells = forAllSystems (pkgs: {
+        default = pkgs.mkShell {
           buildInputs = with pkgs; [
             # Package manager
             pnpm_10
@@ -37,6 +30,6 @@
             fi
           '';
         };
-      };
+      });
     };
 }