Feature: Temporary exemptions

Would it make sense to track on what date someone confirmed the repository was still active, and that way you could confirm that was still the case after X months? As a bonus, we won't have a mysterious, public, keep tag on several of our repositories!

Start by reading the stale-repository detection flow and the GitHub Action configuration, since the issue names no files or tests. Define how a repository records its last activity confirmation, how the X-month period is configured, and what replaces the public keep tag before adding coverage for the full lifecycle.

Written by the indexing model from the issue text.