refactor(output): share the JSON, created-resource and batch tails

Mutations end with a.jsonResult or a.created instead of hand-rolled
--json branches, the two identical jsonResult/writeMutationJSON helpers
become one, and writeBatchJSON checks --json itself so batch commands
share one tail.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/milestone_issues.go

@@ -66,7 +66,7 @@ $ tea milestone issue %[1]s v1.0 12 13 '#14'`, verb), Annotations: map[string]st
 					item, err = service.Edit(cmd.Context(), n, services.IssueEdit{Milestone: &milestone.ID})
 				}
 				return item, itemError(err, target.Repo)
-			}, func(services.Issue) {})
+			}, nil)
 			if len(results) > 0 {
 				changed := make([]string, len(results))
 				for i, item := range results {
@@ -74,10 +74,7 @@ $ tea milestone issue %[1]s v1.0 12 13 '#14'`, verb), Annotations: map[string]st
 				}
 				a.done(cmd, "%s %s %s %s", past, strings.Join(changed, ", "), preposition, milestoneName(target, milestone))
 			}
-			if a.options.json != "" {
-				failures = append(failures, writeBatchJSON(a, cmd, len(numbers), results))
-			}
-			return errors.Join(failures...)
+			return errors.Join(append(failures, writeBatchJSON(a, cmd, len(numbers), results))...)
 		}}
 		command.Flags().BoolVar(&byName, "by-name", false, "Match a numeric milestone argument as a title rather than an ID")
 		milestones, issues := a.completeMilestones(), a.completeIssues("open")