feat: add Vercel AI Gateway as a provider

Adds 'vercel' as a recognized provider throughout the model resolution
system, enabling users with Vercel AI Gateway to use it as a universal
fallback for OpenAI, Anthropic, and Google models.

The gateway transform infers the sub-provider from canonical model names
(claude->anthropic, gpt->openai, gemini->google) and delegates to the
appropriate provider-specific transform, producing model strings like
vercel/anthropic/claude-opus-4.6.

src/cli/model-fallback-types.ts

@@ -11,6 +11,7 @@ export interface ProviderAvailability {
 	zai: boolean
 kimiForCoding: boolean
 	opencodeGo: boolean
+	vercelAiGateway: boolean
 	isMaxPlan: boolean
 }