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/tui-installer.ts

@@ -77,7 +77,7 @@ export async function runTuiInstaller(args: InstallArgs, version: string): Promi
     )
   }
 
-  if (!config.hasClaude && !config.hasOpenAI && !config.hasGemini && !config.hasCopilot && !config.hasOpencodeZen) {
+  if (!config.hasClaude && !config.hasOpenAI && !config.hasGemini && !config.hasCopilot && !config.hasOpencodeZen && !config.hasVercelAiGateway) {
     p.log.warn("No model providers configured. Using opencode/big-pickle as fallback.")
   }