feat(sleep): add HasAnyCompletedSession for feature discovery

Add HasAnyCompletedSession() method to sleep storage to check if any
sleep sessions have been completed. This is used by the feature discovery
notification system to determine when to fire the "first sleep session
complete" notification.

A completed session has both sleep_onset and wake_time set.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

mothership/internal/sleep/storage.go

@@ -539,3 +539,17 @@ func (s *Storage) DeleteOldSessions(days int) (int64, error) {
 
 	return result.RowsAffected()
 }
+
+// HasAnyCompletedSession returns true if at least one sleep session has been completed.
+// A completed session has both sleep_onset and wake_time set.
+func (s *Storage) HasAnyCompletedSession() bool {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	var count int
+	err := s.db.QueryRow(`
+		SELECT COUNT(*) FROM sleep_sessions
+		WHERE sleep_onset > 0 AND wake_time > 0
+	`).Scan(&count)
+	return err == nil && count > 0
+}