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/coding-agent/CHANGELOG.md

@@ -2,12 +2,20 @@
 
 ## [Unreleased]
 
+### Added
+
+- **Google Gemini CLI OAuth provider**: Access Gemini 2.0/2.5 models for free via Google Cloud Code Assist. Login with `/login` and select "Google Gemini CLI". Uses your Google account with rate limits.
+
+- **Google Antigravity OAuth provider**: Access Gemini 3, Claude (sonnet/opus thinking models), and GPT-OSS models for free via Google's Antigravity sandbox. Login with `/login` and select "Antigravity". Uses your Google account with rate limits.
+
 ### Fixed
 
 - **Connection errors not retried**: Added "connection error" to the list of retryable errors so Anthropic connection drops trigger auto-retry instead of silently failing. ([#252](https://github.com/badlogic/pi-mono/issues/252))
 
 - **Thinking level not clamped on model switch**: Fixed TUI showing xhigh thinking level after switching to a model that doesn't support it. Thinking level is now automatically clamped to model capabilities. ([#253](https://github.com/badlogic/pi-mono/issues/253))
 
+- **Cross-model thinking handoff**: Fixed error when switching between models with different thinking signature formats (e.g., GPT-OSS to Claude thinking models via Antigravity). Thinking blocks without signatures are now converted to text with `<thinking>` delimiters.
+
 ## [0.24.5] - 2025-12-20
 
 ### Fixed