Use shlex.quote for remote_cmd in build_ssh_command

Matches the pattern already used in monitor.py's _ssh_batch(); prevents
quoting breakage if shlex.quote(iface) emits single-quoted tokens inside
the remote command string.

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

diagnose.py

@@ -78,7 +78,7 @@ class DiagnosticsRunner:
             f'ssh -o StrictHostKeyChecking=accept-new -o ConnectTimeout=5 '
             f'-o BatchMode=yes -o LogLevel=ERROR '
             f'-o ServerAliveInterval=10 -o ServerAliveCountMax=2 '
-            f'root@{ip_q} \'{remote_cmd}\''
+            f'root@{ip_q} {shlex.quote(remote_cmd)}'
         )
 
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