feat: remove legacy OpenCode runtime

Cut product flows over to Coding/Pi and retain only the migration-owned v1 boundary. Promote supported native optional packages because electron-builder omitted pnpm transitive optional closure from the packaged ASAR.

shared/ai-gateway-error-details.ts

@@ -0,0 +1,6 @@
+export function isAIGatewayUpstreamSaturated(message: string | null | undefined): boolean {
+  const normalized = message?.trim().toLowerCase() ?? '';
+  if (!normalized) return false;
+  return normalized.includes('当前分组上游负载已饱和')
+    || (normalized.includes('upstream') && normalized.includes('saturat'));
+}