[FEATURE]: Fallback models in configuration #8687

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

it would be great if users could specify multiple models in the config (opencode.jsonc / agent/*md) so that an array like

    "$schema": "https://opencode.ai/config.json",
    "default_agent": "plan",
    "model": [
        "anthropic/claude-opus-4-5",
        "google/antigravity-claude-opus-4-5",
        "github-copilot/claude-opus-4-5"
    ],

would be a valid config option.

Why?

• Users that dont have a specific provider connected have a fallback model
• Ideally, opencode detects if provider limits are reached and selects the other model as default when starting a new session
• This way you can continue using e.g. Opus 4.5 without switching the model manually through other providers even if Anthropic's ridiculous rate limits are reached.