polaris.e3sm.init.assemble.AssembleStep.run

AssembleStep.run()

Rebuild the staged tree from empty and stage the README.

The tree is deleted first so that it describes this assembly and not the union of every assembly ever run here. Staging only ever adds links, so without this a changed creation date, mesh name or product set leaves the previous names behind – still resolving, and now pointing at newer content. Rebuilding is a few hundred symlinks, which is cheap next to being unable to trust what is in the tree.