refactor(gateway): remove local ini fallback, configs only via confs

Follow the same convention as the static module: remove local config.ini
fallback and --config/-i command-line argument entirely. All configuration
is now managed exclusively through confs service.

- Delete gateway/example.ini (moved to confs/configs/gateway.ini).
- Remove -i/--input option from boost::program_options; help (-h) and
  version (-v) are the only CLI flags remaining.
- Simplify main.cpp config loading: remove local-file branch, always fetch
  gateway.ini from confs with retry_on_refused=true waiting for confs ready.
- Update _run.sh: no longer passes any arguments; config comes from confs.
- Update README: remove example.ini references, -i flag docs, local file
  mode description, and example.ini from directory structure tree.
- Clarify changelog entry for v0.7.0.

gateway/_run.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 # gateway 启动脚本
-# 默认从 confs 统一配置服务拉取配置（confs 需已启动）。
-# 若需使用本地 ini 文件（开发/调试），传入 -i <path>，例如：./_run.sh -i example.ini
+# 配置统一由 confs 服务管理（confs/configs/gateway.ini），
+# 启动时从 confs（CONFS_HOST:CONFS_PORT，默认 127.0.0.1:8010）拉取配置。
 cd "$(dirname "$0")" || exit
 
 # 等待依赖服务启动（confs/db）