fix: address review feedback for Vercel AI Gateway PR

- Add --vercel-ai-gateway CLI option to install command
- Fix librarian/explore priority: native providers (ZAI, Copilot) now take precedence over Vercel gateway
- Add hasVercelAiGateway to installer no-provider warning conditions
- Add hasVercelAiGateway: false to all test file InstallConfig fixtures
- Fix test expectations for model ID format (claude-opus-4.6 vs claude-opus-4-6)

src/cli/cli-installer.ts

@@ -138,7 +138,8 @@ export async function runCliInstaller(args: InstallArgs, version: string): Promi
     !config.hasOpenAI &&
     !config.hasGemini &&
     !config.hasCopilot &&
-    !config.hasOpencodeZen
+    !config.hasOpencodeZen &&
+    !config.hasVercelAiGateway
   ) {
     printWarning("No model providers configured. Using opencode/big-pickle as fallback.")
   }