Archive labels [GA]

### Value Prop
You can now archive labels to retire them from active use without deleting them or losing any history. Archived labels disappear from pickers, autocomplete, and suggestions immediately, so your label list stays clean and relevant — while remaining applied to all existing issues, pull requests, and discussions. Un-archiving brings a label back at any time, so there's no risk in tidying up.

### Expected Outcome
Repositories with long histories tend to accumulate hundreds of unused labels that clutter the label picker and slow down triage. Archive labels gives maintainers a safe, reversible way to retire labels they no longer need, so contributors can find the right labels quickly. The result is a cleaner labeling workflow and less friction when organizing and prioritizing work.

This roadmap issue names no repository files, tests, or implementation entry points. Start by locating the GitHub label-management code and existing label picker, autocomplete, and suggestion tests, then confirm the scope for archiving and un-archiving labels. Done means archived labels are hidden from active selection while remaining on existing issues, pull requests, and discussions.

索引モデルが issue の本文から書いたものです。