Add Reminder that posts a weekly cleaning-duty reminder to a Telegram
group on a configurable schedule (REMINDER_WEEKDAY/REMINDER_HOUR), in
Europe/Warsaw time, announcing the duty DUTY_AHEAD_DAYS out. Reminders
are disabled unless GROUP_CHAT_ID is set. Wire ReminderConfig parsing
into LoadConfig with validation, document the commands and reminder
config in README, and cover config loading with tests.
Kamil 'Kamaś' Bruchal