docs(store): describe KV answered polls and the shorter life of new-entry hints

The server no longer repeats entry ids in `N` actions, answers up-to-date delta polls
from KV and drops `N` rows after three days. The client never read the entry ids, so
only its type and the design document follow.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>