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/checkout.go

@@ -143,15 +143,12 @@ argument select it, and the branch tracks the head branch, so that git push upda
 			undoRemote()
 			return err
 		}
-		if a.options.json != "" {
-			return a.writeJSON(cmd, result)
-		}
 		if result.Detached {
 			a.done(cmd, "Checked out #%d at %s (detached HEAD)", n, shortSHA(result.Commit))
 		} else {
 			a.done(cmd, "Checked out #%d into %s", n, result.Branch)
 		}
-		return nil
+		return a.jsonResult(cmd, result)
 	}}
 	cmd.Flags().BoolVar(&update, "update", false, "Fast-forward an existing branch to the PR head; reject divergence")
 	cmd.Flags().BoolVar(&detached, "detach", false, "Check out the PR commit without creating or changing a branch")