feat: xAI SuperGrok / X Premium+ OAuth login (xai-oauth)

[GratefulDave]

xAI SuperGrok / X Premium+ OAuth (xai-oauth)

Prime Agent still has no built-in SuperGrok / X Premium+ login. API-key xai works; subscription users cannot /login.

I implemented this as a separate xai-oauth provider (not an overlay on xai):

• Device-code flow via auth.x.ai
• Inference on https://api.x.ai/v1 with X-XAI-Token-Auth: xai-grok-cli
• Catalog cloned from live xAI models (default grok-4.6)
• Flatten exclusive-required MCP object-root anyOf so xAI does not 400 the whole turn

PR (auto-closed by the vouched-author gate): #1639

Branch: GratefulDave:feat/xai-oauth-upstream on current main.

Happy to iterate if a maintainer wants to invite the PR.