nixvim: add nvim-ufo for LSP/treesitter folding

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

modules/home/nixvim/plugins/ui/ufo.nix

@@ -0,0 +1,23 @@
+{ pkgs, ... }: {
+  programs.nixvim = {
+    opts = {
+      foldcolumn = "1";
+      foldlevel = 99;
+      foldlevelstart = 99;
+      foldenable = true;
+    };
+
+    extraPlugins = [
+      pkgs.vimPlugins.nvim-ufo
+      pkgs.vimPlugins.promise-async
+    ];
+
+    extraConfigLua = ''
+      require("ufo").setup({
+        provider_selector = function()
+          return { "treesitter", "indent" }
+        end,
+      })
+    '';
+  };
+}