Identify local providers that support discovery

Listing models here is likely the wrong approach for tools like opencode for local providers, as there are thousands of supported models, they are never deprecated, and displaying any significant amount bogs down UI. For example, only a handful of models are listed here for LM Studio, and they are older models.

Instead, providers should have a "discoverable" field, which can be used to identify that a local provider supports /v1/models, and that this is the preferred method of populating a model list rather than the hardcoded one here. Work is already being done on this model polling at https://github.com/anomalyco/opencode/pull/15732, and this would allow that feature to be easily generalized to other providers.

Start by reviewing the provider definitions and the model-polling work referenced in opencode pull request #15732. Done means local providers can be marked discoverable and model lists use provider discovery instead of relying on large hardcoded lists.