Update the AGU article format to the official 2019 template

[cderv]

Background

The agu_article() format currently bundles agujournal2018.cls. AGU's current submission resources still identify agujournal2019.cls as the primary class and strongly recommend the newest submission template, while allowing older legible templates.

AGU also publishes agujournal2025.cls, but describes it as a newer template that more closely resembles Wiley's typeset output. It should be evaluated separately rather than assumed to be a drop-in replacement for the submission-oriented 2019 class.

Official resources:

• AGU author resources: https://publications.agu.org/author-resource-center/text-requirements/
• AGU 2025 template repository: https://github.com/AGU-Publications/agujournal2025-latex-template

Proposed update

Update the default agu_article() template from agujournal2018.cls to the official agujournal2019.cls, including its current skeleton and submission guidance.

This needs more than renaming the class. In particular:

• agu_article() currently defaults to citation_package = "natbib".
• The existing skeleton teaches \citet and \citep.
• The 2019 class uses apacite conventions and documents \citeA and \cite.
• Track-changes defaults and appendix numbering also changed.

Citation output and existing-document compatibility should therefore receive focused tests.

Current LaTeX compatibility problem

The bundled 2018 class fails to render ordinary tabular environments with the LaTeX 2026-06-01 kernel (Missing $ inserted at \begin{tabular}). The cause is the class's copied \@array implementation, whose center branch uses the math-only \vcenter while current LaTeX uses \vcenter@text.

The official 2019 and 2025 classes retain the same bare \vcenter, so updating the class alone will not fix this failure. The rticles template will need a small backward-compatible workaround (use \vcenter@text when available, otherwise \vcenter) until this is corrected upstream.

Suggested scope

1. Add the narrow LaTeX compatibility workaround and keep the AGU render test enabled.
2. Migrate the submission template to agujournal2019.cls with citation and appendix regression coverage.
3. Evaluate agujournal2025.cls separately, potentially as an additional output format.

[cderv]

cc @eliocamp as you are the original submitter on this template. I'll try to do the update though.