reminders

As someone who has a lot of conflicting time constraints, I would like to be able to get timely reminders of when to practice my skills, a la duo from duolingo threatening you if you skip for too long.

The issue requests timely practice reminders but names no files, tests, entry points, reminder mechanism, or delivery method. First inspect the Shell project structure and existing practice-tracking behavior, then clarify the reminder rules and notification channel before implementation; done should include an agreed behavior and tests for it.

Written by the indexing model from the issue text.