use_vault: Remember a vault path for this R session
Description
Sets options("pensar.vault") so subsequent pensar calls
resolve to path without repeating the argument. Persist by
adding pensar::use_vault("~/wiki") to ~/.Rprofile as
a global default. Both PENSAR_VAULT and a project-local
schema.md found via walk-up will override this option (see
default_vault resolution order).