Add support for Opencode providers

[markokocic]

Provide support for Opencode Go and Opencode Zen providers.

[yuanhao]

Released in v0.9.0 (via #53): ModelConfig::opencode_zen("<model-id>") and ModelConfig::opencode_go("<model-id>") presets that pick the right protocol from the model family (GPT → Responses API, Claude/Qwen → Anthropic Messages with Bearer auth, the rest → Chat Completions). Docs with provider-pairing examples: docs/providers/opencode.md. Gemini-via-Zen isn't supported (Zen serves those over a Google-native endpoint shape). Feedback welcome if a model routes wrong — the gateway-side mapping can change, and {base}/models is the source of truth.