CreateReminderTool

Creates a reminder as a 30-minute calendar event with an alert, inserted into the device's primary calendar (else the first visible calendar with contributor access, see CalendarSupport.findWritableCalendarId) via CalendarContract. Takes a title, a strictly-parsed datetime (yyyy-MM-dd HH:mm, device timezone), 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 writable calendar exists on the device.

Result keys: success, event_id, title, datetime, minutes_before.