refactor(installer): remove dead Antigravity auth plugin code

The installer was writing Antigravity provider config and calling a no-op addAuthPlugins function. Since opencode-antigravity-auth is no longer auto-installed and OpenCode supports native Google/Gemini auth, all Antigravity-related installer code is dead. Gemini detection now checks for native google provider instead.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/cli/config-manager/detect-current-config.ts

@@ -66,7 +66,8 @@ export function detectCurrentConfig(): DetectedConfig {
     return result
   }
 
-  result.hasGemini = plugins.some((p) => p.startsWith("opencode-antigravity-auth"))
+  const providers = openCodeConfig.provider as Record<string, unknown> | undefined
+  result.hasGemini = providers ? "google" in providers : false
 
   const { hasOpenAI, hasOpencodeZen, hasZaiCodingPlan, hasKimiForCoding } = detectProvidersFromOmoConfig()
   result.hasOpenAI = hasOpenAI