Thanks to @bagtoad, this commit refactors the argument to `NewCopilotReplacer(bool)` from being where this is used to what it effect is has.
Because there is already precedence for display name being `<login> (<name>)`, I worried there would be confusion for `Copilot (AI)` being display name for assignees and reviewers but `Copilot` when going to GitHub.com UI. Instead, I renamed the argument based on whether the login is returned / replaced.