fix: add missing packages for zsh aliases and plugins

- home/pkg.nix: add eza, bat, ack, fzf
- modules/core/pkg.nix: add kubectl (for OMZ kubectl plugin)

modules/core/pkg.nix

@@ -5,5 +5,8 @@
     git
     htop
     wget
+
+    # 系统级工具（多个 host 共用）
+    kubectl       # oh-my-zsh kubectl plugin
   ];
 }