add kernel scheduler service on background isolate

Tiered Timer.periodic instances (1min, 10min, 15min, 1hr, midnight)
running on a dedicated Dart isolate. Tick messages sent back to the
main isolate via SendPort and emitted as SchedulerTick events on the
DaemonBus. Extensions subscribe with events.on<SchedulerTick>() and
filter by tier.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/src/scheduler.dart

@@ -0,0 +1,77 @@
+import 'dart:async';
+import 'dart:isolate';
+
+import 'package:clide/kernel/src/events/bus.dart';
+import 'package:clide/kernel/src/events/types.dart';
+import 'package:flutter/foundation.dart';
+
+enum SchedulerTier {
+  oneMinute(Duration(minutes: 1)),
+  tenMinutes(Duration(minutes: 10)),
+  fifteenMinutes(Duration(minutes: 15)),
+  oneHour(Duration(hours: 1)),
+  midnight(Duration(minutes: 1));
+
+  const SchedulerTier(this.interval);
+  final Duration interval;
+}
+
+@immutable
+class SchedulerTick extends ClideEvent {
+  const SchedulerTick({required this.tier});
+  final SchedulerTier tier;
+
+  @override
+  String get subsystem => 'scheduler';
+  @override
+  String get kind => 'tick';
+  @override
+  Map<String, Object?> payload() => {'tier': tier.name};
+}
+
+class SchedulerService {
+  SchedulerService(this._events);
+  final DaemonBus _events;
+
+  Isolate? _isolate;
+  ReceivePort? _port;
+  StreamSubscription<dynamic>? _sub;
+
+  void start() {
+    if (_isolate != null) return;
+    _port = ReceivePort();
+    _sub = _port!.listen((msg) {
+      if (msg is String) {
+        final tier = SchedulerTier.values.firstWhere((t) => t.name == msg);
+        _events.emit(SchedulerTick(tier: tier));
+      }
+    });
+    Isolate.spawn(_isolateEntry, _port!.sendPort).then((iso) => _isolate = iso);
+  }
+
+  static void _isolateEntry(SendPort send) {
+    int lastDay = DateTime.now().day;
+    for (final tier in SchedulerTier.values) {
+      if (tier == SchedulerTier.midnight) {
+        Timer.periodic(const Duration(minutes: 1), (_) {
+          final day = DateTime.now().day;
+          if (day != lastDay) {
+            lastDay = day;
+            send.send(tier.name);
+          }
+        });
+        continue;
+      }
+      Timer.periodic(tier.interval, (_) => send.send(tier.name));
+    }
+  }
+
+  void dispose() {
+    _sub?.cancel();
+    _port?.close();
+    _isolate?.kill(priority: Isolate.immediate);
+    _isolate = null;
+    _port = null;
+    _sub = null;
+  }
+}