A timer_list entity holds many in-memory countdown timers (its items),
mirroring how a to-do list holds many to-do items; its state is the number
of active timers. Adds the base entity platform with start/pause/unpause/
cancel/add_time/remove_time/clear/get services, automation triggers
(timer_started/updated/finished/cancelled), and a websocket subscribe
command. Also adds a local_timer_list helper so timer lists can be created
from the UI.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>