Ability to add a note explaining why user is bypassed from PR limits [GA]

### Value Prop
When adding someone to a repository's pull request bypass list, maintainers can now include a note explaining the reason for the exception. This added context makes it easy for the whole team to understand why each bypass exists — even as the list grows over time. Keeping that history visible helps teams maintain cleaner, more auditable workflows without relying on memory or out-of-band communication.

### Expected Outcome
As bypass lists scale across larger teams and repositories, the intent behind each entry can become unclear, creating confusion and governance gaps. This change gives maintainers a lightweight way to document decisions inline, improving transparency and reducing the overhead of managing access over time. Teams that rely on automated bypass additions will also benefit from automatically generated reasons, ensuring full visibility with no extra effort.

The issue names no files, tests, or entry points. Start by locating the pull request bypass-list implementation and determine where maintainers and automated additions provide and display a note; done means each bypass entry can show its reason.

索引モデルが issue の本文から書いたものです。