seo(site): add /opencode-on-phone/ — targets proven 'setup opencode on phone' query

Google 'opencode android' surfaces a 40+ comment Reddit thread ('best way to
setup opencode on phone') and not us. New high-intent page answers it directly:
HowTo + FAQ + Breadcrumb JSON-LD, 4-step setup, tunnel table, Termux contrast.
Linked from landing footer + in-content, added to sitemap (priority 0.8).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs-site/index.html

@@ -301,7 +301,7 @@ footer a{margin:0 8px}
       <li><b>Session management</b><span>Browse, create, and resume coding sessions on your server.</span></li>
     </ul>
     <p style="margin-top:16px"><a href="features/">Full feature list and use cases →</a></p>
-    <p style="margin-top:8px">Comparing options? See <a href="comparison/">OpenCode Mobile vs. ChatGPT &amp; Copilot</a>, <a href="vs-termux/">OpenCode Mobile vs. Termux</a>, or how to <a href="claude-code-android/">run Claude Code on Android</a>.</p>
+    <p style="margin-top:8px">New here? See <a href="opencode-on-phone/">how to use OpenCode on your phone</a>. Comparing options? Read <a href="comparison/">OpenCode Mobile vs. ChatGPT &amp; Copilot</a>, <a href="vs-termux/">OpenCode Mobile vs. Termux</a>, or how to <a href="claude-code-android/">run Claude Code on Android</a>.</p>
   </div>
 </section>
 
@@ -384,6 +384,7 @@ OPENCODE_SERVER_PASSWORD=yourpassword opencode serve --hostname 0.0.0.0 --port 4
     <p>
       <a href="download/">Download</a>
       <a href="guide/">Setup guide</a>
+      <a href="opencode-on-phone/">OpenCode on your phone</a>
       <a href="features/">Features</a>
       <a href="comparison/">Comparison</a>
       <a href="vs-termux/">vs Termux</a>