Warn or error when --cloud-config is passed

[sudomateo]

Context

The cloud provider registration func (internal/provider/provider.go) discards the io.Reader for the config file — configuration is entirely env-var based (OXIDE_HOST/OXIDE_TOKEN/OXIDE_PROJECT). A user passing --cloud-config gets silence: their config file does nothing and there's no hint why.

Scope

• Log a warning (or return an error) when the config reader is non-empty.
• Document in the README that configuration is env-var based and --cloud-config is unsupported.

Done when

Passing --cloud-config produces visible feedback instead of silence.

[sudomateo]

We will actually want to support both the configuration file and the environment variables.