feat: location-based reminders (geofence arrive/leave triggers)

[BRO3886]

Summary

Add support for location-based reminders (geofence triggers) — arrive/leave alerts tied to a geographic coordinate. This is a real feature in Reminders.app that rem currently can't express.

EventKit Public API

Location reminders are fully supported by the public EventKit API — no private ReminderKit bridge needed for core functionality:

• EKStructuredLocation — holds title, geoLocation (CLLocation), and radius (meters)
• EKAlarm.structuredLocation — attach a structured location to any alarm
• EKAlarm.proximity — EKAlarmProximityEnter (arrive) or EKAlarmProximityLeave (depart)

EKStructuredLocation *loc = [EKStructuredLocation locationWithTitle:@"Grocery Store"];
loc.geoLocation = [[CLLocation alloc] initWithLatitude:37.3318 longitude:-122.0312];
loc.radius = 200.0; // meters; 0 = system default

EKAlarm *alarm = [[EKAlarm alloc] init];
alarm.structuredLocation = loc;
alarm.proximity = EKAlarmProximityEnter;
[reminder addAlarm:alarm];

Private ReminderKit extras (optional, not required)

From issue #27's exploration:

• REMAlarmVehicleTrigger — trigger on CarPlay vehicle arrival/departure (no public API equivalent)
• REMStructuredLocation — richer than EKStructuredLocation (has address, routing, mapKitHandle)
• REMAlarmContactTrigger — trigger when messaging a contact

Only vehicle triggers are unique to the private API. Standard geofence arrive/leave works entirely through public EventKit.

Implementation plan

go-eventkit (library layer)

1. Add Location *AlarmLocation to the Alarm struct
2. New AlarmLocation struct: Title, Latitude, Longitude, Radius, Proximity (arrive/leave/none)
3. In bridge_darwin.m: read/write alarm.structuredLocation and alarm.proximity
4. Add -framework CoreLocation to LDFLAGS (new framework dependency for CLLocation)
5. Wire through CreateReminderInput and UpdateReminderInput

rem (CLI layer)

• rem add --location "37.33,-122.03" --on-arrive or --on-leave
• rem show displays location info when present
• rem list --has-location filter (nice-to-have)
• rem update --location to add/change/remove location trigger
• Export/import roundtrip for location data

UX considerations

• Coordinate input: raw lat,lng is not ergonomic but is the simplest path. Geocoding (address → coordinates) would need CLGeocoder which is async and requires network — adds meaningful complexity
• Radius: default to system minimum (0), allow --radius 200 override in meters
• Proximity: --on-arrive (default) and --on-leave flags, similar to how --remind-me works for time-based alarms

Permissions & testing

• Location-based reminders require Location Services permission in addition to Reminders — second TCC prompt on macOS
• Geofences only fire on physical device movement — not testable in headless/CI environments
• Unit tests can verify struct serialization; integration tests limited to create/read (can't trigger the geofence)

References

• EKStructuredLocation docs
• Managing location-based reminders
• EKAlarm.proximity
• Issue Enhancement ideas from private ReminderKit API exploration #27 — original private API exploration that catalogued location trigger classes