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/utils/oauth/storage.ts

@@ -0,0 +1,108 @@
+/**
+ * OAuth credential storage for ~/.pi/agent/oauth.json
+ */
+
+import { chmodSync, existsSync, mkdirSync, readFileSync, writeFileSync } from "fs";
+import { homedir } from "os";
+import { dirname, join } from "path";
+
+export interface OAuthCredentials {
+	type: "oauth";
+	refresh: string;
+	access: string;
+	expires: number;
+	enterpriseUrl?: string;
+	projectId?: string;
+	email?: string;
+}
+
+export interface OAuthStorage {
+	[provider: string]: OAuthCredentials;
+}
+
+export type OAuthProvider = "anthropic" | "github-copilot" | "google-gemini-cli" | "google-antigravity";
+
+/**
+ * Get the path to the OAuth credentials file
+ */
+export function getOAuthPath(): string {
+	return join(homedir(), ".pi", "agent", "oauth.json");
+}
+
+/**
+ * Ensure the config directory exists
+ */
+function ensureConfigDir(): void {
+	const configDir = dirname(getOAuthPath());
+	if (!existsSync(configDir)) {
+		mkdirSync(configDir, { recursive: true, mode: 0o700 });
+	}
+}
+
+/**
+ * Load all OAuth credentials from ~/.pi/agent/oauth.json
+ */
+export function loadOAuthStorage(): OAuthStorage {
+	const filePath = getOAuthPath();
+	if (!existsSync(filePath)) {
+		return {};
+	}
+
+	try {
+		const content = readFileSync(filePath, "utf-8");
+		return JSON.parse(content);
+	} catch {
+		return {};
+	}
+}
+
+/**
+ * Save all OAuth credentials to ~/.pi/agent/oauth.json
+ */
+function saveOAuthStorage(storage: OAuthStorage): void {
+	ensureConfigDir();
+	const filePath = getOAuthPath();
+	writeFileSync(filePath, JSON.stringify(storage, null, 2), "utf-8");
+	chmodSync(filePath, 0o600);
+}
+
+/**
+ * Load OAuth credentials for a specific provider
+ */
+export function loadOAuthCredentials(provider: string): OAuthCredentials | null {
+	const storage = loadOAuthStorage();
+	return storage[provider] || null;
+}
+
+/**
+ * Save OAuth credentials for a specific provider
+ */
+export function saveOAuthCredentials(provider: string, creds: OAuthCredentials): void {
+	const storage = loadOAuthStorage();
+	storage[provider] = creds;
+	saveOAuthStorage(storage);
+}
+
+/**
+ * Remove OAuth credentials for a specific provider
+ */
+export function removeOAuthCredentials(provider: string): void {
+	const storage = loadOAuthStorage();
+	delete storage[provider];
+	saveOAuthStorage(storage);
+}
+
+/**
+ * Check if OAuth credentials exist for a provider
+ */
+export function hasOAuthCredentials(provider: string): boolean {
+	return loadOAuthCredentials(provider) !== null;
+}
+
+/**
+ * List all providers with OAuth credentials
+ */
+export function listOAuthProviders(): string[] {
+	const storage = loadOAuthStorage();
+	return Object.keys(storage);
+}