Resolves a ModelProfile for a given provider and model.

Resolution order:

1. Registry (models.dev cache) — the single source of truth
2. Overlay merge — provider/model-specific fields from priv/overlays.json that models.dev doesn't carry (thinking_param, preserve_reasoning, provider_routing)
3. Provider-level default — weakest fallback for local providers not in models.dev (ollama, lmstudio, vllm)
4. Global default — unknown everything