Convert a repository's Branch Protections to Rulesets [GA]

### Value Prop
Repository admins can now convert all legacy branch protections to rulesets in a single guided flow, complete with a dry-run preview that shows exactly which rulesets will be created, flags redundant protections, and surfaces any settings that don't map directly. Each converted ruleset preserves the original protection's behavior, and admins choose whether to activate rules immediately or run them in evaluation mode first. This makes adopting rulesets straightforward and low-risk, with no gaps in rule coverage along the way.

### Expected Outcome
Teams that have been putting off migrating to rulesets due to the manual, one-by-one effort involved can now complete the transition quickly and confidently at the repository level. By removing the cognitive load of manually re-creating each protection, we want to accelerate adoption of rulesets as the modern, more powerful standard for branch governance across organizations.

从 roadmap issue 的预期结果开始：在 repository 层面将旧版 branch protections 引导式转换为 rulesets。该 payload 提到了 dry-run 预览、冗余保护检测、未映射设置、行为保持和评估模式，但没有提供实现文件、测试或入口点；在确定工作范围之前，先识别负责的产品界面。

由索引模型根据 Issue 内容生成。