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>

core-reminders/build.gradle.kts

@@ -0,0 +1,32 @@
+// core-reminders — pure-Kotlin reminder-lead plumbing shared across the family:
+// the lead-time unit model (ReminderUnit / decomposeReminderMinutes), the
+// per-target override model (ReminderOverride + resolution helpers) and the
+// stored-format codec (ReminderOverrideCodec, separators configurable so each
+// app keeps its on-disk format). No Android, no third-party deps — a plain JVM
+// library, like core-time; each app layers its own DataStore + string labels on
+// top.
+plugins {
+    alias(libs.plugins.kotlin.jvm)
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_17
+    targetCompatibility = JavaVersion.VERSION_17
+}
+
+kotlin {
+    compilerOptions {
+        jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17
+    }
+}
+
+dependencies {
+    testImplementation(libs.junit.jupiter.api)
+    testRuntimeOnly(libs.junit.jupiter.engine)
+    testRuntimeOnly(libs.junit.platform.launcher)
+    testImplementation(libs.truth)
+}
+
+tasks.test {
+    useJUnitPlatform()
+}