feat: implement gt mq reject command for manual MR rejection

- Add gt mq command group with reject subcommand
- Add FindMR method to locate MRs by ID or branch
- Add RejectMR method to reject MRs with reason
- Add notifyWorkerRejected for optional mail notification
- Add tests for rejection state transitions

Closes gt-svi.5

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/refinery/manager.go

@@ -551,6 +551,88 @@ Thank you for your contribution!`,
 	router.Send(msg)
 }
 
+// Common errors for MR operations
+var (
+	ErrMRNotFound = errors.New("merge request not found")
+)
+
+// FindMR finds a merge request by ID or branch name.
+// Returns nil if not found.
+func (m *Manager) FindMR(idOrBranch string) (*MergeRequest, error) {
+	queue, err := m.Queue()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, item := range queue {
+		// Match by ID
+		if item.MR.ID == idOrBranch {
+			return item.MR, nil
+		}
+		// Match by branch name (with or without polecat/ prefix)
+		if item.MR.Branch == idOrBranch {
+			return item.MR, nil
+		}
+		if "polecat/"+idOrBranch == item.MR.Branch {
+			return item.MR, nil
+		}
+		// Match by worker name (partial match for convenience)
+		if strings.Contains(item.MR.ID, idOrBranch) {
+			return item.MR, nil
+		}
+	}
+
+	return nil, ErrMRNotFound
+}
+
+// RejectMR manually rejects a merge request.
+// It closes the MR with rejected status and optionally notifies the worker.
+// Returns the rejected MR for display purposes.
+func (m *Manager) RejectMR(idOrBranch string, reason string, notify bool) (*MergeRequest, error) {
+	mr, err := m.FindMR(idOrBranch)
+	if err != nil {
+		return nil, err
+	}
+
+	// Verify MR is open or in_progress (can't reject already closed)
+	if mr.IsClosed() {
+		return nil, fmt.Errorf("%w: MR is already closed with reason: %s", ErrClosedImmutable, mr.CloseReason)
+	}
+
+	// Close with rejected reason
+	if err := mr.Close(CloseReasonRejected); err != nil {
+		return nil, fmt.Errorf("failed to close MR: %w", err)
+	}
+	mr.Error = reason
+
+	// Optionally notify worker
+	if notify {
+		m.notifyWorkerRejected(mr, reason)
+	}
+
+	return mr, nil
+}
+
+// notifyWorkerRejected sends a rejection notification to a polecat.
+func (m *Manager) notifyWorkerRejected(mr *MergeRequest, reason string) {
+	router := mail.NewRouter(m.workDir)
+	msg := &mail.Message{
+		From:    fmt.Sprintf("%s/refinery", m.rig.Name),
+		To:      fmt.Sprintf("%s/%s", m.rig.Name, mr.Worker),
+		Subject: "Merge request rejected",
+		Body: fmt.Sprintf(`Your merge request has been rejected.
+
+Branch: %s
+Issue: %s
+Reason: %s
+
+Please review the feedback and address the issues before resubmitting.`,
+			mr.Branch, mr.IssueID, reason),
+		Priority: mail.PriorityNormal,
+	}
+	router.Send(msg)
+}
+
 // findTownRoot walks up directories to find the town root.
 func findTownRoot(startPath string) string {
 	path := startPath