chore: 建立 staging 晋级流程 / Establish staging promotion flow

[YihuiLu]

Summary

• 新建长期 staging 集成分支，用于 feature/fix PR 和社区测试 / Add the long-lived staging integration branch for feature/fix PRs and community testing
• main 只接受 staging PR 晋级 / Promote to main only from staging PRs
• 同步 CI、Branch Guard、保护脚本和贡献文档 / Update CI, Branch Guard, protection script, and contribution docs

Validation

• git diff --check
• bash -n .github/scripts/apply-branch-protection.sh
• Ruby YAML parse for .github/workflows/ci.yml and .github/workflows/branch-guard.yml

Develop branch

• origin/develop has no unique commits and is behind main by 23 commits / origin/develop 没有独有提交且落后 main 23 个提交
• It should be replaced by staging instead of merged / 应由 staging 替代，不需要合并。