refactor: 移除独立 hub/proxy/executor/gateway crate，统一为 gateway tunnel 架构

- 删除 aether-hub、aether-proxy 独立项目及其 Dockerfile/配置
- 删除 crates/aether-executor 和 crates/aether-gateway 全部模块
- 新增 apps/ 目录作为应用入口
- 将 hub 概念重构为 gateway tunnel transport
- 将 executor 重构为 execution runtime
- 新增 tunnel.rs 合约定义和 testkit tunnel/execution_runtime 模块
- 更新 Python 服务层和测试适配新架构命名

apps/aether-gateway/src/api/ai/mod.rs

@@ -0,0 +1,9 @@
+mod claude;
+mod gemini;
+mod openai;
+mod registry;
+
+pub(crate) use registry::{
+    admin_default_body_rules_for_signature, admin_endpoint_signature_parts, mount_ai_routes,
+    normalize_admin_endpoint_signature, public_api_format_local_path,
+};