Add Grok (xAI) provider#570

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BryanTegomoh wants to merge 3 commits intosteipete:mainfrom

BryanTegomoh:feature/grok-provider

Commits on Mar 19, 2026

Add Grok (xAI) provider with Management API billing support
BryanTegomoh
committed
Fix env var names and align with xAI docs and CodexBar patterns
BryanTegomoh
committed

Commits on Mar 20, 2026

Fix descriptor registry, billing schema, and review feedback
BryanTegomoh
committed