fix(release): 默认使用 Kubernetes 生产通道

原因：旧 Compose 生产环境已退役，默认通道仍指向 Compose 会在每次发布时先命中无效数据库。\n\n影响：发布和部署脚本默认读取 Kubernetes release helper；仍可通过显式 AI_GATEWAY_DEPLOY_MODE=compose 操作遗留环境。\n\n验证：bash -n、ShellCheck 和 manual-release-test 全部通过。

scripts/deploy-production-release.sh

@@ -3,7 +3,7 @@ set -euo pipefail
 
 root=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
 production_host=${AI_GATEWAY_PRODUCTION_HOST:-root@110.42.51.33}
-deploy_mode=${AI_GATEWAY_DEPLOY_MODE:-compose}
+deploy_mode=${AI_GATEWAY_DEPLOY_MODE:-kubernetes}
 case $deploy_mode in
   compose) default_remote_helper=/usr/local/sbin/easyai-ai-gateway-release ;;
   kubernetes) default_remote_helper=/usr/local/sbin/easyai-ai-gateway-cluster-release ;;
@@ -23,6 +23,9 @@ Usage:
 
 This command changes production. Run it only after a successful publish
 command and an explicit user request to release or deploy to production.
+
+AI_GATEWAY_DEPLOY_MODE defaults to kubernetes. Set it to compose only for an
+explicit legacy-environment operation.
 EOF
 }