[FEATURE]: Allow Go plan users to restrict which models the client can enable

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Plan: OpenCode Go

As a Go plan subscriber, the client fetches and lists all supported models, and any of them can be enabled/selected. It's easy to accidentally pick a model that isn't covered (or is billed differently) by my plan, which leads to mistakes — sometimes cost-related, sometimes just wasted time when the model behaves unexpectedly.

The Zen plan already supports restricting which models the client can use via the management/admin dashboard. Please provide the same capability for the Go plan: let subscribers configure, in the admin dashboard (zen.opencode.ai), an allowlist of models the client is permitted to enable. The client should then only show/enable those configured models.

Expected behavior:

1. Go plan users can configure an allowed model list in the admin dashboard (same as Zen plan).
2. The client only lists/enables models from that allowlist.