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

crates/aether-testkit/Cargo.toml

@@ -10,9 +10,7 @@ description = "Shared integration test helpers for Aether Rust services"
 async-stream.workspace = true
 aether-data.workspace = true
 aether-contracts.workspace = true
-aether-executor.workspace = true
 aether-gateway.workspace = true
-aether-hub.workspace = true
 aether-http.workspace = true
 aether-runtime.workspace = true
 axum.workspace = true