Fix tree: remove double icons, add .nojekyll for _doc/ serving

Remove chevron-down icons from tree toggles — folder/folder-open
swap is sufficient visual indicator. Add .nojekyll so GitHub Pages
serves underscore-prefixed _doc/ directory. Remove unused .tree-chevron
CSS rules.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: markc

docs/.nojekyll

@@ -618,9 +618,6 @@
     .tree-item .lucide, .tree-toggle .lucide { flex-shrink: 0; width: 1em; height: 1em; color: var(--fg-muted); }
     .tree-item.active [data-lucide], .tree-item.active .lucide { color: var(--accent); }
 
-    .tree-chevron { transition: transform var(--transition-fast); }
-    .tree-branch.collapsed .tree-chevron { transform: rotate(-90deg); }
-
     .tree-children { display: grid; grid-template-rows: 1fr; transition: grid-template-rows 0.3s ease-in-out; }
     .tree-branch.collapsed .tree-children { grid-template-rows: 0fr; }
     .tree-children > div { overflow: hidden; min-height: 0; }

docs/base.css

@@ -72,14 +72,12 @@ <h1><a class="brand" href="./">Dual Carousel <span>Sidebars</span></a></h1>
                     <div class="tree">
                         <div class="tree-branch">
                             <div class="tree-toggle" style="--tree-depth:0">
-                                <i data-lucide="chevron-down" class="tree-chevron"></i>
                                 <i data-lucide="folder-open"></i>
                                 <span>docs</span>
                             </div>
                             <div class="tree-children"><div>
                                 <div class="tree-branch">
                                     <div class="tree-toggle" style="--tree-depth:1">
-                                        <i data-lucide="chevron-down" class="tree-chevron"></i>
                                         <i data-lucide="folder-open"></i>
                                         <span>_doc</span>
                                     </div>