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/main/java/de/jeanlucmakiola/agendula/domain/DayWindow.kt

@@ -1,18 +0,0 @@
-package de.jeanlucmakiola.agendula.domain
-
-import java.time.ZoneId
-import kotlin.time.Instant
-
-/** Local-day boundaries used by the smart-list predicates. Pure + testable. */
-object DayWindow {
-
-    /** Returns `[startOfToday, startOfTomorrow)` in [zone] for the instant [now]. */
-    fun today(now: Instant, zone: ZoneId): Pair<Instant, Instant> {
-        val date = java.time.Instant.ofEpochMilli(now.toEpochMilliseconds())
-            .atZone(zone)
-            .toLocalDate()
-        val start = date.atStartOfDay(zone).toInstant().toEpochMilli()
-        val end = date.plusDays(1).atStartOfDay(zone).toInstant().toEpochMilli()
-        return Instant.fromEpochMilliseconds(start) to Instant.fromEpochMilliseconds(end)
-    }
-}

app/src/main/java/de/jeanlucmakiola/agendula/ui/common/DateTimeFormat.kt

@@ -1,28 +0,0 @@
-package de.jeanlucmakiola.agendula.ui.common
-
-import java.time.ZoneId
-import java.time.format.DateTimeFormatter
-import java.time.format.FormatStyle
-import kotlin.time.Instant
-
-/**
- * Locale-aware display formatting for the [kotlin.time.Instant]s the domain
- * uses. Kept tiny and dependency-free; the data layer stores instants, the UI
- * renders them in the device's zone and locale.
- */
-private val dateFormatter: DateTimeFormatter = DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM)
-private val timeFormatter: DateTimeFormatter = DateTimeFormatter.ofLocalizedTime(FormatStyle.SHORT)
-
-private fun Instant.atSystemZone() =
-    java.time.Instant.ofEpochMilli(toEpochMilliseconds())
-        .atZone(ZoneId.systemDefault())
-
-/** Medium localized date, e.g. "18 Jun 2026". */
-fun Instant.formatDate(): String = atSystemZone().format(dateFormatter)
-
-/** Short localized time, e.g. "14:30". */
-fun Instant.formatTime(): String = atSystemZone().format(timeFormatter)
-
-/** Date alone for all-day items, otherwise date + time. */
-fun Instant.formatDateTime(allDay: Boolean): String =
-    if (allDay) formatDate() else "${formatDate()} · ${formatTime()}"