feat(xai): track SuperGrok weekly limit + API usage quota

Fetch OAuth quota from cli-chat-proxy billing, GetGrokCreditsConfig weekly
window, and settings plan label so the dashboard matches grok.com usage.

open-sse/providers/registry/xai.js

@@ -25,6 +25,15 @@ export default {
     clientId: "b1a00492-073a-47ea-816f-4c329264a828",
     tokenUrl: "https://auth.x.ai/oauth2/token",
     refreshUrl: "https://auth.x.ai/oauth2/token",
+    // OAuth-only SuperGrok quota surfaces:
+    // - url: monthly API usage allotment (JSON)
+    // - creditsUrl: weekly SuperGrok limit (grpc-web)
+    // - settingsUrl: plan label (subscription_tier_display)
+    usage: {
+      url: "https://cli-chat-proxy.grok.com/v1/billing",
+      creditsUrl: "https://grok.com/grok_api_v2.GrokBuildBilling/GetGrokCreditsConfig",
+      settingsUrl: "https://cli-chat-proxy.grok.com/v1/settings",
+    },
   },
   models: [
     { id: "grok-4", name: "Grok 4" },
@@ -57,4 +66,7 @@ export default {
     endpoint: "https://api.x.ai/v1/responses",
     pricingUrl: "https://x.ai/api#pricing",
   },
+  features: {
+    usage: true,
+  },
 };