feat: add Google Antigravity authentication support

- Add 'antigravity' as new auth choice in onboard and configure wizards
- Implement Google Antigravity OAuth flow using loginAntigravity from pi-ai
- Update writeOAuthCredentials to accept any OAuthProvider (not just 'anthropic')
- Add schema sanitization for Google Cloud Code Assist API to fix tool call errors
- Default model set to google-antigravity/claude-opus-4-5 after successful auth

The schema sanitization removes unsupported JSON Schema keywords (patternProperties,
const, anyOf, etc.) that Google's Cloud Code Assist API doesn't understand.

src/commands/onboard-auth.ts

@@ -1,7 +1,7 @@
 import fs from "node:fs/promises";
 import path from "node:path";
 
-import type { OAuthCredentials } from "@mariozechner/pi-ai";
+import type { OAuthCredentials, OAuthProvider } from "@mariozechner/pi-ai";
 import { discoverAuthStorage } from "@mariozechner/pi-coding-agent";
 
 import { resolveClawdisAgentDir } from "../agents/agent-paths.js";
@@ -9,7 +9,7 @@ import type { ClawdisConfig } from "../config/config.js";
 import { CONFIG_DIR } from "../utils.js";
 
 export async function writeOAuthCredentials(
-  provider: "anthropic",
+  provider: OAuthProvider,
   creds: OAuthCredentials,
 ): Promise<void> {
   const dir = path.join(CONFIG_DIR, "credentials");