model overrides reject OpenRouter and AWS Bedrock #134

Hi, I may be misunderstanding the intended scope of model overrides.

models and fallbacks can use openrouter and aws-bedrock, but model_overrides and model_family_overrides appear to accept only opencode-go and opencode-zen.

For example, this family override fails validation:

{
  "model_family_overrides": {
    "sonnet": {
      "provider": "openrouter",
      "model_id": "test-model"
    }
  }
}

model_family_overrides["sonnet"] has invalid provider "openrouter"
(must be "opencode-go" or "opencode-zen")

The error appears to come from the provider check in validateOverrideMap in internal/config/loader.go. I also found the OpenRouter dispatch path in internal/client/opencode.go, so I was unsure whether the override restriction is intentional or whether I am missing another constraint.

Are exact and family overrides meant to support only OpenCode Go and Zen?

If possible, it would be helpful if OpenRouter and AWS Bedrock could also be used with both exact and family overrides. This would allow override routing to use the same providers that are already available in models and fallbacks.

Related: #44, #109, #116, #120