feat: Sprint 18 — file preview auto-close, thinking display, workspace tree

- File preview auto-close: clearPreview() extracted as named function
  and called from loadDir(). Navigating directories (breadcrumbs, up
  button, folder clicks) now automatically closes the right panel
  file preview instead of leaving stale content visible.

- Thinking/reasoning display: assistant messages with structured content
  arrays containing type=thinking or type=reasoning blocks now render
  as collapsible gold-themed cards above the response text. Collapsed
  by default, click header to expand. Works with Claude extended thinking
  and o3 reasoning tokens when preserved in the message array.

- Workspace tree view (Issue #22): directories expand/collapse in-place
  with toggle arrows. Single-click toggles, double-click navigates
  (breadcrumb view). Subdirectory contents fetched lazily and cached.
  Indentation shows nesting depth. Empty directories show "(empty)".
  S._expandedDirs tracks open state, S._dirCache caches fetched entries.

Tests: 295 passed, 23 pre-existing failures, 0 regressions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/boot.js

@@ -43,14 +43,16 @@ $('importFileInput').onchange=async(e)=>{
   }
 };
 // btnRefreshFiles is now panel-icon-btn in header (see HTML)
-$('btnClearPreview').onclick=()=>{
-  $('previewArea').classList.remove('visible');
-  $('previewImg').src='';
-  $('previewMd').innerHTML='';
-  $('previewCode').textContent='';
-  $('previewPathText').textContent='';
-  $('fileTree').style.display='';
-};
+function clearPreview(){
+  const pa=$('previewArea');if(pa)pa.classList.remove('visible');
+  const pi=$('previewImg');if(pi)pi.src='';
+  const pm=$('previewMd');if(pm)pm.innerHTML='';
+  const pc=$('previewCode');if(pc)pc.textContent='';
+  const pp=$('previewPathText');if(pp)pp.textContent='';
+  const ft=$('fileTree');if(ft)ft.style.display='';
+  _previewCurrentPath='';_previewCurrentMode='';_previewDirty=false;
+}
+$('btnClearPreview').onclick=clearPreview;
 // workspacePath click handler removed -- use topbar workspace chip dropdown instead
 $('modelSelect').onchange=async()=>{
   if(!S.session)return;