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/types.ts

@@ -11,6 +11,7 @@ export interface InstallArgs {
   zaiCodingPlan?: BooleanArg
 kimiForCoding?: BooleanArg
   opencodeGo?: BooleanArg
+  vercelAiGateway?: BooleanArg
   skipAuth?: boolean
 }
 
@@ -24,6 +25,7 @@ export interface InstallConfig {
   hasZaiCodingPlan: boolean
   hasKimiForCoding: boolean
   hasOpencodeGo: boolean
+  hasVercelAiGateway: boolean
 }
 
 export interface ConfigMergeResult {
@@ -44,4 +46,5 @@ export interface DetectedConfig {
   hasZaiCodingPlan: boolean
   hasKimiForCoding: boolean
   hasOpencodeGo: boolean
+  hasVercelAiGateway: boolean
 }