flake: add coding-agent validation workflow

shells/dotnix.nix

@@ -3,15 +3,22 @@ _: {
     {
       pkgs,
       config,
+      lib,
       ...
     }:
     {
       devShells.dotnix = pkgs.mkShell {
         packages = [
           config.treefmt.build.wrapper
+          config.packages.dotfiles-check
 
+          pkgs.actionlint
           pkgs.git
           pkgs.gitleaks
+          pkgs.mcp-nixos
+          pkgs.nix-fast-build
+          pkgs.nix-tree
+          pkgs.nixd
           pkgs.pre-commit
 
           # sops-nix / age
@@ -22,6 +29,8 @@ _: {
           # YubiKey for sops editing
           pkgs.age-plugin-yubikey
           pkgs.yubikey-manager
+        ]
+        ++ lib.optionals pkgs.stdenv.isLinux [
           pkgs.pcsc-tools
         ];