refactor(nix): split dev shell into dedicated flake

CHANGELOG.md

@@ -18,6 +18,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 - Migrate Nix flake configuration from flake-utils to flake-parts. (#52)
 - Add autonomy and communication guidance for AI agents. (#42)
 - Document `nix develop` devShell usage in contributing guide. (#43)
+- Split the Nix devShell into `nix/devshell/flake.nix`, keep the root package flake on `flake-parts`, and source coding agents from `llm-agents.nix`. (#56)
 - Refine one-line descriptions (README, Nix flake metadata, Homebrew formula, and GitHub repository description). (#43)
 
 ### Changed

CONTRIBUTING.md

@@ -48,10 +48,10 @@ We welcome contributions of all kinds and appreciate various types of contributi
 If you use Nix, you can enter a Rust-ready dev shell:
 
 ```sh
-nix develop
+nix develop ./nix/devshell
 ```
 
-This devShell installs `rustup` and `rust-analyzer`, coding agents (`opencode`, `codex`, `claude-code`, `gemini-cli`), and `zsh`, then switches to `zsh`.
+This devShell installs `rustup` and `rust-analyzer`, coding agents (`codex`, `claude-code`, `gemini-cli`, `ccusage`, `opencode`) from `llm-agents.nix`, and `zsh`, then switches to `zsh`.
 
 ### Building `ignore`
 

flake.nix

@@ -3,7 +3,6 @@
 
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
-    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
     flake-parts = {
       url = "github:hercules-ci/flake-parts";
       inputs.nixpkgs-lib.follows = "nixpkgs";
@@ -13,22 +12,15 @@
 
   outputs =
     inputs@{
-      nixpkgs,
-      nixpkgs-unstable,
       flake-parts,
       systems,
       ...
     }:
     flake-parts.lib.mkFlake { inherit inputs; } {
       systems = import systems;
       perSystem =
-        { system, ... }:
+        { pkgs, ... }:
         let
-          pkgs = import nixpkgs { inherit system; };
-          pkgs-unstable = import nixpkgs-unstable {
-            inherit system;
-            config.allowUnfree = true;
-          };
           ignorePackage = pkgs.rustPlatform.buildRustPackage {
             pname = "ignore";
             version = "0.4.0";
@@ -51,30 +43,12 @@
           };
         in
         {
+          formatter = pkgs.nixfmt-tree;
+
           packages = {
             default = ignorePackage;
             ignore = ignorePackage;
           };
-
-          devShells.default = pkgs.mkShell {
-            packages =
-              with pkgs;
-              [
-                rustup
-                rust-analyzer
-                zsh
-                opencode
-              ]
-              ++ (with pkgs-unstable; [
-                codex
-                claude-code
-                gemini-cli
-              ]);
-
-            shellHook = ''
-              exec zsh
-            '';
-          };
         };
     };
 }