feat: files pane like VS Code: tree, tabs, go to file

- Explorer tree with inline expand/collapse and keyboard navigation, git
  colours, change dots, dimmed gitignored entries (fs-list now flags them),
  file-type badges, resizable explorer.
- Editor tabs: preview tab semantics, instant switching with scroll kept,
  in-place reload when Claude changes an open file.
- Go to file: devdesk attach --fs-find (git ls-files or capped find) and a
  fuzzy-ranked palette (CommandPalette gained custom search/async lists).
- Per-session memory of folders and tabs; empty state with recents.

Closes #37

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/index.template.html

@@ -93,17 +93,24 @@
   <section class="lt-frame dd-files" id="files" aria-label="Files" hidden>
     <div class="dd-files-resizer" id="files-resizer" role="separator" aria-orientation="vertical" aria-label="Resize the files pane" tabindex="0"></div>
     <div class="dd-files-head">
-      <div class="dd-files-tabs" role="tablist">
-        <button type="button" role="tab" data-files-tab="files" aria-selected="true">Files</button>
-        <button type="button" role="tab" data-files-tab="changes" aria-selected="false">Changes <span class="dd-files-count"></span></button>
-      </div>
+      <button type="button" class="dd-icon-btn dd-explorer-toggle" title="Show / hide the explorer" aria-label="Toggle explorer">☰</button>
+      <div class="dd-tabs" role="tablist" aria-label="Open files"></div>
       <span class="dd-files-actions">
+        <button type="button" class="dd-icon-btn dd-files-find" title="Go to file (Ctrl+P)" aria-label="Go to file">⌕</button>
         <button type="button" class="dd-icon-btn dd-files-refresh" title="Refresh" aria-label="Refresh">↻</button>
         <button type="button" class="dd-icon-btn dd-files-close" title="Close (Esc)" aria-label="Close files">✕</button>
       </span>
     </div>
-    <nav class="dd-files-crumbs" aria-label="Path"></nav>
-    <div class="dd-files-body" tabindex="-1"></div>
+    <div class="dd-files-main">
+      <div class="dd-explorer">
+        <div class="dd-explorer-views" role="tablist">
+          <button type="button" role="tab" data-files-view="explorer" aria-selected="true">Explorer</button>
+          <button type="button" role="tab" data-files-view="changes" aria-selected="false">Changes <span class="dd-files-count"></span></button>
+        </div>
+        <div class="dd-explorer-body" role="tree" aria-label="Files"></div>
+      </div>
+      <div class="dd-editor-area"></div>
+    </div>
   </section>
 
   <div class="dd-backdrop" id="backdrop" hidden></div>