chore(sponsors): drop Unity2.ai partner from README, presets and i18n

Remove the Unity2.ai sponsor entry from the four README files (en, zh, ja,
de) and the provider preset across all eight preset files (claude, codex,
gemini, opencode, openclaw, hermes, claudeDesktop, grokBuild). Also drop
the matching partnerPromotion.unity2 string in the zh/en/ja/zh-TW locales.

Historical CHANGELOG and release-notes entries are left untouched.

src/config/grokBuildProviderPresets.ts

@@ -173,18 +173,6 @@ export const grokBuildProviderPresets: GrokBuildProviderPreset[] = [
     partnerPromotionKey: "runapi",
     icon: "runapi",
   },
-  {
-    name: "Unity2.ai",
-    websiteUrl: "https://unity2.ai",
-    apiKeyUrl: "https://unity2.ai/register?source=ccs",
-    auth: grokAuth(),
-    config: grokPresetConfig("Unity2.ai", "https://api.unity2.ai"),
-    endpointCandidates: ["https://api.unity2.ai"],
-    category: "aggregator",
-    isPartner: true,
-    partnerPromotionKey: "unity2",
-    icon: "unity2",
-  },
   {
     name: "Shengsuanyun",
     nameKey: "providerForm.presets.shengsuanyun",