Reject an empty pr edit title Gitea would ignore

cmd/agentpr/main.go

@@ -163,6 +163,11 @@ func newPREditCmd() *cobra.Command {
 			// leave the title as it is, not blank it.
 			var opts agent.EditPROptions
 			if cmd.Flags().Changed("title") {
+				// Gitea ignores an empty title, so sending one would report
+				// success while changing nothing.
+				if title == "" {
+					return fmt.Errorf("--title cannot be empty: a title can be set but not cleared")
+				}
 				opts.Title = &title
 			}
 			if cmd.Flags().Changed("body") {