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/core/model-resolver.ts

@@ -14,6 +14,8 @@ export const defaultModelPerProvider: Record<KnownProvider, string> = {
 	anthropic: "claude-sonnet-4-5",
 	openai: "gpt-5.1-codex",
 	google: "gemini-2.5-pro",
+	"google-gemini-cli": "gemini-2.5-pro",
+	"google-antigravity": "gemini-3-pro-high",
 	"github-copilot": "gpt-4o",
 	openrouter: "openai/gpt-5.1-codex",
 	xai: "grok-4-fast-non-reasoning",