V2: xAI Grok OAuth (SuperGrok subscription) login not supported #34778

Summary

The v2 branch does not support logging into xAI with the xAI Grok OAuth (SuperGrok subscription) flow. V1 supports it; V2 has no xAI login method registered.

Part of the systemic V2 provider-login gap (see #34765 for the OpenAI/ChatGPT case).

Evidence

• packages/core/src/plugin/provider/xai.ts — no ctx.integration.transform / no method.update, so no OAuth login method is registered. Catalog/SDK only.
• Repo-wide, only openai and opencode register OAuth methods in packages/core/src/plugin/provider/.

How V1 does it (reference)

packages/opencode/src/plugin/xai.ts (XaiAuthPlugin):

• Registers an OAuth method labeled "xAI Grok OAuth (SuperGrok Subscription)" (PKCE + device/callback flow).
• loader returns a custom fetch that, for OAuth credentials:
  • single-flight refreshes the rotating refresh_token and persists via client.auth.set,
  • sets authorization: Bearer <access> and User-Agent: opencode/<version>,
  • intentionally does not override baseURL (relies on @ai-sdk/xai default https://api.x.ai/v1).

What V2 needs

1. Register the xAI OAuth login method in xai.ts.
2. Request-time auth: inject Authorization: Bearer <access> for the OAuth credential (token refresh should route through V2's integration refresh on the method registration, not an inline fetch refresh).

Ideally this is expressed through the shared credential-aware request hook proposed in #34765 (so it works for both the AI SDK and the native @opencode-ai/llm paths) rather than re-porting V1's fetch monkey-patch.

Environment

• Branch: v2
• Status: verified against source; not runtime-reproduced (requires a SuperGrok subscription).