chore: 切换 OGScope 分支策略到 staging / Switch OGScope branch policy to staging

[YihuiLu]

Summary

• 将 OGScope 长期集成分支从 develop 切换为 staging / Switch the long-lived OGScope integration branch from develop to staging
• 让 main 只接受 staging PR 晋级，并让 staging 接受 feature/fix PR / Restrict main promotion to staging PRs and route feature/fix work through staging
• 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

Notes

• origin/develop has no unique commits and is behind main; it should be replaced by staging rather than merged / origin/develop 没有独有提交且落后 main，应由 staging 替代，不需要合并。

[YihuiLu]

Bootstrap note: this PR correctly fails the new main-source guard because it is not from staging. Replacing it with a staging -> main PR. / 引导说明：该 PR 因不是 staging 来源而被新守卫正确拒绝，将改用 staging -> main PR。