refactor: move optional modules to modules/optional/

- Move virtualbox-guest.nix and vscode-server.nix from modules/server/ to modules/optional/
- Create modules/optional/README.md with usage docs
- Update profiles/virtualbox.nix and profiles/wsl.nix import paths
- Update modules/server/README.md to reflect removal
- Update root README.md directory tree with optional/ entry

modules/optional/README.md

@@ -0,0 +1,22 @@
+# 📦 modules/optional
+
+选装模块目录——非默认启用，按需在 profile 中引用。
+
+## 🌲 目录树
+```text
+📦 modules/optional
+├─ 📦 virtualbox-guest.nix  # VirtualBox Guest 增强（加入 wheel 组、拖拽支持）
+├─ 🧠 vscode-server.nix     # VS Code Server + direnv + nix-ld
+└─ 📝 README.md
+```
+
+## 用法
+
+在对应 profile 或 host 配置中添加 import：
+
+```nix
+imports = [
+  ../modules/optional/virtualbox-guest.nix
+  ../modules/optional/vscode-server.nix
+];
+```