fix(install): 默认切回 GitHub release，atomgit 镜像当前不可用

[bigbrother666sh]

实测 atomgit 三处全坏

• raw 文件接口（6 种格式全试）返回 gitcode SPA HTML（3576 字节），curl 拿不到脚本 → bash -c 跑 HTML
• API /api/v1/repos/... 被 CloudWAF 拦 418（任何 UA）
• v5.6.0 release 资产 404（只同步了 tag，没同步 140MB tarball；旧 v5.5.2 wiseflow-.zip 还在但新 xiaobei- 命名未同步）

GitHub 全通：raw=200/72KB 真脚本，release asset=200/147MB 真包。

修法

• install.sh / install.ps1 默认 GitHub（mirror 空），--atomgit/-Atomgit 改 opt-in（待 atomgit infra 修复后重新作为国内默认），--github/-GitHub 保留向后兼容
• README：GitHub 一行命令为唯一推荐（macOS/Linux bash + Windows PowerShell），升级段同步；atomgit 标注「修复中暂不可用」
• install.sh 加 --atomgit flag + 用法帮助更新

验证

• bash -n 通过；默认（无 flag）→ XIAOBEI_MIRROR 空（走 GitHub）✓；--atomgit → atomgit mirror ✓
• GitHub raw 实拉 72423 字节 #!/bin/bash ✓

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