fix(agent): restart systemd service on reinstall instead of no-op start

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

agent/install.sh

@@ -142,8 +142,13 @@ WantedBy=multi-user.target
 EOF
   systemctl daemon-reload
   systemctl enable usite-agent
-  systemctl start usite-agent
-  ok "Servicio systemd 'usite-agent' instalado y activo"
+  if systemctl is-active --quiet usite-agent; then
+    systemctl restart usite-agent
+    ok "Servicio systemd 'usite-agent' reiniciado con nueva configuración"
+  else
+    systemctl start usite-agent
+    ok "Servicio systemd 'usite-agent' instalado y activo"
+  fi
   STARTED=1
 
 # 2) openrc (Alpine, algunos VPS)