Log and return PersistSession error in check command

A failed file write was silently ignored, causing attendance updates
to be lost with no indication of failure to cron or the log.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/cli/cmd/check.go

@@ -121,7 +121,10 @@ will attend the next session.`,
 				}
 			}
 
-			data.PersistSession(session)
+			if err := data.PersistSession(session); err != nil {
+				log.Default().Printf("ERROR persisting session: %s", err)
+				return err
+			}
 		}
 
 		return nil