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.

Start with the roadmap issue's expected outcome: a repository-level guided conversion from legacy branch protections to rulesets. The payload names dry-run preview, redundant-protection detection, unmapped settings, behavior preservation, and evaluation mode, but provides no implementation files, tests, or entry points; identify the owning product surface before work can be scoped.

索引モデルが issue の本文から書いたものです。