CreateReminderTool
Creates a reminder as a 30-minute calendar event with an alert, inserted into the device's primary calendar via CalendarContract. Takes a title, a datetime (yyyy-MM-dd HH:mm), and optional minutes_before (default 10) for the alert lead time.
Requires both Manifest.permission.READ_CALENDAR and Manifest.permission.WRITE_CALENDAR (checked at runtime; returns an error if either is missing). Fails if no calendar exists on the device.
Result keys: success, event_id, title, datetime, minutes_before.