refactor: move sudo to core, lxc SSH to profile, clean up old lxc config

- sudo (wheelNeedsPassword) → modules/core/core.nix (全局)
- LXC SSH override (PermitRootLogin, PasswordAuth) → profiles/lxc.nix
- modules/server/ssh.nix: use mkDefault so profiles can override
- Delete modules/optional/lxc.nix
- Clean up hosts/nixos-lxc/configuration.nix: remove redundant SSH/sudo/users
  (now handled by core + server + profile chain)
- Update modules/optional/README.md

hosts/nixos-lxc/configuration.nix

@@ -21,32 +21,10 @@
   # 由宿主机处理 fstrim
   services.fstrim.enable = false;
 
-  services.openssh = {
-    enable = true;
-    openFirewall = true;
-    settings = {
-      PermitRootLogin = "yes";
-      PasswordAuthentication = true;
-    };
-  };
-
-  # 用户账户配置
-  users.users.luochen570 = {
-    isNormalUser = true;
-    extraGroups = [ "wheel" "networkmanager" ];
-  };
-
-  security.sudo.wheelNeedsPassword = false;
-
   services.resolved = {
     extraConfig = ''
       Cache=true
       CacheFromLocalhost=true
     '';
   };
-
-  nix.settings.substituters = [ "https://mirrors.tuna.tsinghua.edu.cn/nix-channels/store" ];
-  nix.settings.experimental-features = [ "nix-command" "flakes" ];
-
-  system.stateVersion = "25.05";
 }