Reject --before combined with --first

NewCursorFromFlags validated --first/--last, --after/--before, and
--after/--last as mutually exclusive, but missing the symmetric
--first/--before guard. Passing --first with --before fell into the
backward branch, where size defaulted from the unset --last and the
direction silently flipped to Tail, discarding --first and reversing
pagination. Reject the combination up front like the other invalid
pairings.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/proboctl/cmdutil/paginate.go

@@ -87,6 +87,10 @@ func NewCursorFromFlags[F page.OrderField](
 		return nil, fmt.Errorf("--after cannot be combined with --last")
 	}
 
+	if pf.Before != "" && pf.First > 0 {
+		return nil, fmt.Errorf("--before cannot be combined with --first")
+	}
+
 	var (
 		size     int
 		from     *page.CursorKey