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/provider-availability.ts

@@ -13,6 +13,7 @@ export function toProviderAvailability(config: InstallConfig): ProviderAvailabil
 		zai: config.hasZaiCodingPlan,
 kimiForCoding: config.hasKimiForCoding,
 		opencodeGo: config.hasOpencodeGo,
+		vercelAiGateway: config.hasVercelAiGateway,
 		isMaxPlan: config.isMax20,
 	}
 }
@@ -27,6 +28,7 @@ export function isProviderAvailable(provider: string, availability: ProviderAvai
 		"zai-coding-plan": availability.zai,
 "kimi-for-coding": availability.kimiForCoding,
 		"opencode-go": availability.opencodeGo,
+		vercel: availability.vercelAiGateway,
 	}
 	return mapping[provider] ?? false
 }