add missing enum constraint

Author: tonytrg

pkg/github/pullrequests.go

@@ -216,6 +216,7 @@ func ListPullRequests(getClient GetClientFn, t translations.TranslationHelperFun
 			),
 			mcp.WithString("direction",
 				mcp.Description("Sort direction"),
+				mcp.Enum("asc", "desc"),
 			),
 			WithPagination(),
 		),