feat(create): add --disk-size flag

[theFong]

Summary

• add a --disk-size <GB> flag to brev create
• apply the requested size to every resolved fallback instance type
• allow an explicit disk size to override launchable storage defaults
• reject zero, negative, NaN, and infinite disk sizes

Why

The create API already accepts diskStorage, and search/piped workflows already populate InstanceSpec.DiskGB. However, explicitly selected instance types were created with DiskGB: 0, so agents and users had no direct way to request variable storage while using --type.

Impact

Agents can now deploy a GPU with provider-supported variable storage directly:

brev create my-gpu --type g5.xlarge --disk-size 1000

The value is sent to the create API as a Gi quantity. Provider-side storage constraints continue to determine which sizes are supported.

Validation

• go test ./pkg/cmd/gpucreate
• go test -race ./pkg/cmd/gpucreate
• go vet ./pkg/cmd/gpucreate
• golangci-lint run ./pkg/cmd/gpucreate