feat: expose native reminder creation

[SebastianBoehler]

Summary

• Activate create_reminder in the shared native-tool contract introduced by the Port native device control tools to Flutter agent #30 branch.
• Add the Dart tool schema with title, ISO-8601 time, optional type, and optional repeat.
• Route Android reminder creation through the copied ReminderManager, with capability checks for notification and exact-alarm availability.
• Align iOS with the same native-tool method by scheduling one-time local notification reminders and returning explicit errors for unsupported type/repeat combinations.
• Add focused Dart tests for reminder tool exposure, supported native-tool filtering, and routing.

Stacked on #33 / issue-30-native-tool-routing.
Refs #31.

Validation

• git diff --check
• git diff --cached --check

Could not run flutter test because Flutter is not installed in this runtime. Could not run Android Kotlin compilation because Java/JAVA_HOME is missing in this runtime.

Manual QA Needed

• Android: one-time reminder, repeating reminder, exact-alarm permission denial, notification permission denial, reboot/reschedule smoke test.
• iOS: notification permission denial and successful one-time local notification scheduling.