fix(cli): migrate Gemini models to explicit antigravity- prefix for quota routing

All Gemini model references now use the `antigravity-` prefix to ensure explicit
routing to Antigravity quota pools instead of relying on legacy `-preview` suffix
disambiguation. This approach prevents potential breakage if Google removes the
`-preview` suffix in future versions.

Updates include:
- config-manager: Updated Gemini model assignments with antigravity- prefix
- config-manager.test.ts: Updated test assertions to match new naming convention
- install.ts: Updated config summary display to show antigravity-prefixed model name

Migration follows opencode-antigravity-auth plugin v1.2.7+ guidance for explicit
quota routing configuration.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/cli/config-manager.test.ts

@@ -11,9 +11,9 @@ describe("config-manager ANTIGRAVITY_PROVIDER_CONFIG", () => {
     expect(models).toBeTruthy()
 
     const required = [
-      "gemini-3-pro-high",
-      "gemini-3-pro-low",
-      "gemini-3-flash",
+      "antigravity-gemini-3-pro-high",
+      "antigravity-gemini-3-pro-low",
+      "antigravity-gemini-3-flash",
     ]
 
     for (const key of required) {