`brev create` should have two subcommands, `brev create gpu` and `brev create cpu`, mirroring `brev search`

[brycelelbach]

brev create defaults to the cheapest GPU node. There's no syntax for "give me the cheapest CPU node".

Just as we split brev search into brev search [gpu] and brev search cpu, we should split brev create into brev create [gpu] and brev create cpu. All of our docs, skills, and helpfiles should explicitly qualify the command with either gpu or cpu.

For compatibility with existing code, brev create should default to brev create gpu, but we should consider this "legacy". brev create with no specs should continue defaulting to the cheapest GPU, again for legacy. However, given the advent of claws and free Brev CPU instances, we may want to reconsider this in the future.

See #375