The guard .onAttach() uses before it creates anything. A directory counts
as a project root if it holds an .Rproj file, a _quarto.yml, a
DESCRIPTION, a .git directory, or all three mariner folders.
Details
The last of those is what makes a directory mariner_setup_project()
scaffolded count as a project afterwards, without it also having to be an
RStudio or Quarto one.
It is exported so the attach behaviour can be checked: if library(mariner)
did not create the folders you expected, this says why.