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/src/modes/interactive/components/oauth-selector.ts

@@ -1,6 +1,6 @@
+import { loadOAuthCredentials } from "@mariozechner/pi-ai";
 import { Container, isArrowDown, isArrowUp, isEnter, isEscape, Spacer, TruncatedText } from "@mariozechner/pi-tui";
 import { getOAuthProviders, type OAuthProviderInfo } from "../../../core/oauth/index.js";
-import { loadOAuthCredentials } from "../../../core/oauth/storage.js";
 import { theme } from "../theme/theme.js";
 import { DynamicBorder } from "./dynamic-border.js";