events: reminders + digests from the agenda + 14 tests

reminders.sx bridges calendar + durable rosters to notify: ev/occurrence-
reminders (one per booked attendee, fires lead before start, idempotency key
occ-key/recipient/lead), ev/agenda-reminders (sorted by fire-at),
ev/due-reminders (fire-at <= now), ev/reminder->msg (notify wire shape),
ev/agenda-digest + ev/agenda-for-p. 196/196 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/events/conformance.conf

@@ -40,6 +40,7 @@ PRELOADS=(
   lib/flow/api.sx
   lib/events/notify.sx
   lib/events/api.sx
+  lib/events/reminders.sx
 )
 
 SUITES=(
@@ -49,4 +50,5 @@ SUITES=(
   "booking:lib/events/tests/booking.sx:(ev-booking-tests-run!)"
   "ticket:lib/events/tests/ticket.sx:(ev-ticket-tests-run!)"
   "notify:lib/events/tests/notify.sx:(ev-notify-tests-run!)"
+  "reminders:lib/events/tests/reminders.sx:(ev-reminders-tests-run!)"
 )