Refactor `zstash` for improved maintainability

### Request criteria

- [x] I searched the [zstash GitHub Discussions](https://github.com/E3SM-Project/zstash/discussions) to find a similar question and didn't find it.
- [x] I searched the [zstash documentation](https://e3sm-project.github.io/zstash).
- [x] This issue does not match the other templates (i.e., it is not a bug report, documentation request, feature request, or a question.)

### Issue description

@TonyB9000 and I made substantial improvements to `zstash`'s structure in #416 in an effort to better support transfer tracking, but we've also noted things beyond the scope of that PR that could further improve `zstash`'s maintainability. I'm starting this issue to track ideas for such a future refactor (whose purpose would be solely better maintainability, not just in service of fixing a bug).

The issue does not identify files, tests, entry points, or a concrete refactor. Begin by reviewing PR #416 and the current zstash structure; a follow-up should define a bounded maintainability goal and clear completion criteria before work starts.

Written by the indexing model from the issue text.