feat(opencode): model neutral provider gateway options #529

Context

The neutral provider facts and catalog contract intentionally carries routing, credentials, model metadata, and recommendations only. It has no harness-neutral representation for gateway request options.

Deferred behavior

The retired direct USAi OpenCode configuration set request timeout, stream chunk timeout, and LiteLLM proxy compatibility under provider.usai.options. The provider-neutral renderer preserves the equivalent cost-key translation and recommendations/default roles, but cannot project these provider-owned OpenCode options without either expanding the neutral schemas with OpenCode/USAI-specific fields or reintroducing provider-owned config.

Follow-up

Design a neutral, trusted provider capability/configuration contract for renderer-owned gateway options, then add validated inputs and parity coverage. Until then, the renderer must continue to emit only baseURL and the environment-variable reference from trusted facts.