core-reminders: shared reminder-lead plumbing (ReminderUnit, override model + codec)

Pure-Kotlin module, drawn from the duplicate reminder logic in Agendula and
Calendula: the lead-time unit model (ReminderUnit / decomposeReminderMinutes),
the per-target override model (ReminderOverride + reminderLeadFor /
applyReminderOverride) and the stored-format codec (ReminderOverrideCodec).

The codec's separators are configurable so each app keeps its on-disk format
(Agendula stores id:minutes, Calendula id=minutes) without a data migration.

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

settings.gradle.kts

@@ -32,6 +32,7 @@ dependencyResolutionManagement {
 rootProject.name = "floret-kit"
 
 include(":core-time")
+include(":core-reminders")
 include(":core-crash")
 include(":identity")
 include(":components")