Fix actor isolation crash on macOS 26 (Tahoe)

[nickj]

Summary

Fixes the crash reported in #1 where any command accessing reminders fails with:

Incorrect actor executor assumption; expected 'RemindCore.RemindersStore' executor.

Root Cause

The fetchReminders callback from EventKit.fetchReminders(matching:) runs on an arbitrary thread. Inside that callback, we were calling self.item(from: reminder) which accesses actor-isolated state (self.calendar). Swift 6's strict concurrency checking enforces this at runtime on macOS 26.

Fix

Capture the calendar before entering the callback closure, then use it directly inside. This avoids accessing actor-isolated state from the non-isolated callback context.

Before:

eventStore.fetchReminders(matching: predicate) { reminders in
  let mapped = (reminders ?? []).map { reminder in
    self.item(from: reminder)  // ❌ Accesses actor from wrong executor
  }
  continuation.resume(returning: mapped)
}

After:

let cal = self.calendar  // ✅ Capture on actor executor
eventStore.fetchReminders(matching: predicate) { reminders in
  let mapped = (reminders ?? []).map { reminder in
    ReminderItem(
      // ... use captured `cal` instead of self.calendar
    )
  }
  continuation.resume(returning: mapped)
}

Testing

• All existing tests pass
• Manually verified on macOS 26.2 (Tahoe) with Swift 6.2.3
• remindctl list, remindctl today, remindctl upcoming all work correctly

Fixes #1

[steipete]

Thank you! Reviewing...

[steipete]

Thanks! Closing in favor of #3 (merged). #3 keeps the callback boundary Sendable-only and does ReminderItem construction back on the actor.