feat(trace-workbench): reveal the session's jsonl in the file manager

POST /api/sessions/:id/reveal resolves the file server-side (execFile,
argv array — no shell) and opens the platform file manager; the header
button shows the full path on hover.

research/trace-workbench/index.html

@@ -36,6 +36,7 @@
             <span class="replay-pill">replay</span>
             <strong id="conversationTitle">Loading session...</strong>
             <span id="sessionCrumb"></span>
+            <button id="revealSession" class="reveal-btn" type="button" title="">在 Finder 中显示</button>
           </div>
           <div class="segmented large">
             <button class="active" data-main-view="conversation">Chat</button>