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/workspace.js

@@ -12,6 +12,7 @@ async function loadDir(path){
     S.currentDir=path||'.';
     const data=await api(`/api/list?session_id=${encodeURIComponent(S.session.session_id)}&path=${encodeURIComponent(path)}`);
     S.entries=data.entries||[];renderBreadcrumb();renderFileTree();
+    if(typeof clearPreview==='function')clearPreview();
   }catch(e){console.warn('loadDir',e);}
 }