docs(readme): add a clickable YouTube intro thumbnail above the teaser

A prominent clickable poster-frame linking to the 26-min intro video (what
it is, a walkthrough, and how-to), placed above the existing teaser GIF. The
2:33 silent .mp4 walkthrough link is kept unchanged below it.

README.md

@@ -6,6 +6,14 @@ AI Agents Company - A virtual organization of 20 AI agents + 1 human CEO, design
   <img src="docs/images/run.png" alt="RoboCo control panel: the task tree for a feature, showing Board → Main PM → Backend / Frontend / UX/UI cells → developer subtasks, with live lifecycle statuses (completed, in progress, awaiting PM review, paused) and real GitHub PRs (#59–#62)." width="100%">
 </p>
 
+<p align="center">
+  <a href="https://www.youtube.com/watch?v=t1QNqJgBmkM">
+    <img src="https://img.youtube.com/vi/t1QNqJgBmkM/maxresdefault.jpg" alt="Watch the 26-minute RoboCo intro on YouTube — what it is, a walkthrough, and how to use it" width="80%">
+  </a>
+  <br>
+  <sub>▶ <b><a href="https://www.youtube.com/watch?v=t1QNqJgBmkM">Watch the 26-min intro on YouTube</a></b> — what it is, a walkthrough, and how to use it</sub>
+</p>
+
 <p align="center">
   <img src="docs/videos/panel-teaser.gif" alt="Twelve-second looping preview of the RoboCo control panel — the org tree, a task in progress, and an approval queue." width="80%">
   <br>