Draw core-time from floret-kit via submodule + composite build

Phase 0 of the shared-kit plan: floret-kit is embedded as a git submodule and
wired in with Gradle includeBuild("floret-kit"), so the app depends on
de.jeanlucmakiola.floret:core-time and Gradle substitutes the local source
module (built from source — F-Droid-safe). Proves the submodule + composite-build
pipeline before any design layers move.

- Remove app copies of DayWindow + Instant date/time formatting (now in
  core-time); repoint imports to de.jeanlucmakiola.floret.time.
- DateTimeField gains an explicit import (was same-package).
- CI checkouts fetch submodules recursively.
- Clean composite build + unit tests + debug assemble green.

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

app/src/test/java/de/jeanlucmakiola/agendula/domain/DayWindowTest.kt

@@ -1,35 +0,0 @@
-package de.jeanlucmakiola.agendula.domain
-
-import com.google.common.truth.Truth.assertThat
-import org.junit.jupiter.api.Test
-import java.time.ZoneId
-import kotlin.time.Instant
-
-class DayWindowTest {
-
-    @Test
-    fun `today spans exactly 24h in UTC and contains now`() {
-        val zone = ZoneId.of("UTC")
-        val now = Instant.fromEpochMilliseconds(1_750_000_000_000L)
-        val (start, end) = DayWindow.today(now, zone)
-
-        assertThat(end.toEpochMilliseconds() - start.toEpochMilliseconds()).isEqualTo(86_400_000L)
-        assertThat(now >= start).isTrue()
-        assertThat(now < end).isTrue()
-        // In UTC, midnight aligns to a multiple of one day since the epoch.
-        assertThat(start.toEpochMilliseconds() % 86_400_000L).isEqualTo(0L)
-    }
-
-    @Test
-    fun `start is local midnight in a non-UTC zone`() {
-        val zone = ZoneId.of("Europe/Berlin")
-        val now = Instant.fromEpochMilliseconds(1_750_000_000_000L)
-        val (start, end) = DayWindow.today(now, zone)
-
-        val startLocal = java.time.Instant.ofEpochMilli(start.toEpochMilliseconds()).atZone(zone)
-        assertThat(startLocal.hour).isEqualTo(0)
-        assertThat(startLocal.minute).isEqualTo(0)
-        assertThat(now >= start).isTrue()
-        assertThat(now < end).isTrue()
-    }
-}