chore(auth): remove deprecated models and ChatGPT hotfix

- Remove gemini-3-pro-medium and gemini-3-flash-lite (deprecated in antigravity v1.2.7)
- Remove CHATGPT_HOTFIX_REPO variable and setupChatGPTHotfix() function
- Update CODEX_PROVIDER_CONFIG to modern variants system

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

src/cli/config-manager.test.ts

@@ -12,10 +12,8 @@ describe("config-manager ANTIGRAVITY_PROVIDER_CONFIG", () => {
 
     const required = [
       "gemini-3-pro-high",
-      "gemini-3-pro-medium",
       "gemini-3-pro-low",
       "gemini-3-flash",
-      "gemini-3-flash-lite",
     ]
 
     for (const key of required) {