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/tui-install-prompts.ts

@@ -110,6 +110,16 @@ export async function promptInstallConfig(detected: DetectedConfig): Promise<Ins
   })
   if (!opencodeGo) return null
 
+  const vercelAiGateway = await selectOrCancel({
+    message: "Do you have a Vercel AI Gateway API key?",
+    options: [
+      { value: "no", label: "No", hint: "Will use other configured providers" },
+      { value: "yes", label: "Yes", hint: "Universal proxy for OpenAI, Anthropic, Google, etc." },
+    ],
+    initialValue: initial.vercelAiGateway,
+  })
+  if (!vercelAiGateway) return null
+
   return {
     hasClaude: claude !== "no",
     isMax20: claude === "max20",
@@ -120,5 +130,6 @@ export async function promptInstallConfig(detected: DetectedConfig): Promise<Ins
     hasZaiCodingPlan: zaiCodingPlan === "yes",
     hasKimiForCoding: kimiForCoding === "yes",
     hasOpencodeGo: opencodeGo === "yes",
+    hasVercelAiGateway: vercelAiGateway === "yes",
   }
 }