Add Google Gemini CLI and Antigravity OAuth providers

- Add google-gemini-cli provider: free Gemini 2.0/2.5 via Cloud Code Assist
- Add google-antigravity provider: free Gemini 3, Claude, GPT-OSS via sandbox
- Move OAuth infrastructure from coding-agent to ai package
- Fix thinking signature handling for cross-model handoff
- Fix OpenAI message ID length limit (max 64 chars)
- Add GitHub Copilot overflow pattern detection
- Add OAuth provider tests for context overflow and streaming

packages/ai/src/index.ts

@@ -2,11 +2,12 @@ export * from "./agent/index.js";
 export * from "./models.js";
 export * from "./providers/anthropic.js";
 export * from "./providers/google.js";
-export * from "./providers/google-cloud-code-assist.js";
+export * from "./providers/google-gemini-cli.js";
 export * from "./providers/openai-completions.js";
 export * from "./providers/openai-responses.js";
 export * from "./stream.js";
 export * from "./types.js";
+export * from "./utils/oauth/index.js";
 export * from "./utils/overflow.js";
 export * from "./utils/typebox-helpers.js";
 export * from "./utils/validation.js";