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/publish-release-images.sh

@@ -6,7 +6,7 @@ cd "$root"
 
 components=auto
 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 ;;
@@ -29,6 +29,7 @@ changes production. When the user requested a production release, continue
 with scripts/deploy-production-release.sh without requesting confirmation again.
 
 Environment:
+  AI_GATEWAY_DEPLOY_MODE          Production target (default: kubernetes)
   AI_GATEWAY_PRODUCTION_HOST       SSH target used for read-only release status
   AI_GATEWAY_REMOTE_RELEASE_HELPER Fixed status/deploy helper on the server
   AI_GATEWAY_IMAGE_REGISTRY        Registry namespace