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。

tests/release/manual-release-test.sh

@@ -5,6 +5,15 @@ root=$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)
 tmp=$(mktemp -d)
 trap 'rm -rf "$tmp"' EXIT
 
+for release_script in \
+  "$root/scripts/publish-release-images.sh" \
+  "$root/scripts/deploy-production-release.sh"; do
+  grep -F "deploy_mode=\${AI_GATEWAY_DEPLOY_MODE:-kubernetes}" "$release_script" >/dev/null || {
+    echo "production release script does not default to Kubernetes: $release_script" >&2
+    exit 1
+  }
+done
+
 new_repo() {
   local name=$1
   local repo=$tmp/$name