Fix SSH known_hosts permission error when running as www-data

Add -o UserKnownHostsFile=/dev/null to remoteExec so SSH doesn't
attempt to write to /var/www/.ssh/known_hosts (permission denied).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

panel/lib/ProxyManager.php

@@ -80,7 +80,7 @@ class ProxyManager {
         if (!$r['host']) return 'no remote host configured';
         return shell_exec(
             'sshpass -p ' . escapeshellarg($r['pass']) .
-            ' ssh -o StrictHostKeyChecking=no -o ConnectTimeout=5 ' .
+            ' ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o ConnectTimeout=5 ' .
             escapeshellarg($r['user'] . '@' . $r['host']) . ' ' .
             escapeshellarg($cmd) . ' 2>&1'
         ) ?? '';