Function detectProviderByModel

detectProviderByModel(model: string): ProviderId | null
Detect provider from a model name using signature matching.

Detection priority (first match wins):
1. OpenRouter: model contains / — OpenRouter uses org/model notation (e.g. "anthropic/claude-opus-4", "meta-llama/llama-3.1-8b"). This check runs BEFORE signature matching so that e.g. "anthropic/claude-opus-4" routes to OpenRouter, not the native Anthropic API.
2. Native providers: model name matches a known signature
  - OpenAI: gpt-, o1-, o3-, chatgpt-
  - Anthropic: claude-*
  - xAI: grok-*
3. No match: returns null — caller falls back to local if LOCAL_BASE_URL is set.
Parameters
- model: string
  Model identifier
Returns ProviderId | null
Provider ID if detected, null otherwise
Example
```
detectProviderByModel('gpt-4-turbo');              // 'openai'
detectProviderByModel('claude-3-opus');            // 'anthropic'
detectProviderByModel('grok-2');                   // 'xai'
detectProviderByModel('anthropic/claude-opus-4');  // 'openrouter' (slash notation)
detectProviderByModel('openai/gpt-4o');            // 'openrouter' (slash notation)
detectProviderByModel('meta-llama/llama-3.1-8b'); // 'openrouter' (slash notation)
detectProviderByModel('Qwen3-4B-MLX');             // null (unknown → local fallback)
```
- Defined in providers/detection.ts:142