`schema use` hardcoded `gbrain-base` (and a fixed bundled list), so other
bundled packs could not be selected by name. Use the shared BUNDLED_PACK_NAMES
set and resolve `<name>.yaml` generically so every bundled pack activates.
Closes#1574
Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
