seo(site): ship comparison, vs-termux, claude-code-android pages + internal links

3 long-tail pages targeting 'opencode vs chatgpt/copilot', 'vs termux',
'claude code on android'. Added to sitemap, footer nav, in-content links
from landing + guide + troubleshooting. All have canonical + install CTAs.

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

docs-site/troubleshooting/index.html

@@ -177,6 +177,7 @@ footer a{margin:0 8px}
     </p>
     <p>
       See also: the <a href="../guide/">setup guide</a> · <a href="../download/">download &amp; install</a> ·
+      <a href="../vs-termux/">OpenCode Mobile vs Termux</a> · <a href="../claude-code-android/">using Claude on Android</a> ·
       <a href="../">overview</a>.
     </p>
   </div>