chore(sponsors): remove Quantum Proxies (no longer sponsoring)

Quantum Proxies is no longer a sponsor. Remove their Studio Partners entry
from the README and delete the banner/logo assets.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -427,18 +427,6 @@ Please remove secrets, cookies, private tokens, and customer data from logs befo
       <a href="https://go.nodemaven.com/webclaw">NodeMaven</a>.
     </td>
   </tr>
-  <tr>
-    <td width="340" align="center">
-      <a href="https://quantumproxies.net/?utm_source=webclaw&utm_medium=github&utm_campaign=sponsor">
-        <img src="./assets/sponsors/quantum-proxies-banner.png" alt="Quantum Proxies" width="300" />
-      </a>
-    </td>
-    <td>
-      <strong>Quantum Proxies</strong> provides fast, reliable residential and ISP proxy infrastructure for developers running large-scale extraction workloads.
-      Get 20% off any plan with code <code>WEBCLAW20</code> at
-      <a href="https://quantumproxies.net/?utm_source=webclaw&utm_medium=github&utm_campaign=sponsor">quantumproxies.net</a>.
-    </td>
-  </tr>
   <tr>
     <td width="340" align="center">
       <a href="https://proxy-seller.com/?partner=KXMQNNLIGHXR4B">