The RejectMR function was modifying the in-memory MR object but never
persisting the change to beads storage. This caused rejected MRs to
continue showing in the queue with status "open".
Fix: Call beads.CloseWithReason() to properly close the MR bead before
updating the in-memory state.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>