Promote AgentCompany-New to the repository root and retire the old app

The legacy application - its src, tests, landing site, promo assets, docs,
release tooling, CI workflows, and root configs - leaves the tree; it stays
reachable in history. Everything under AgentCompany-New moves up unchanged:
same package name, so the production userData/database path is untouched,
and e2e/build scripts already resolved paths relative to the project. The
developer cred.txt stays gitignored at its new location. Full verify:ci
passes from the new root.

src/main/services/provider-background.ts

@@ -0,0 +1,28 @@
+import type { ProviderObservedRateLimitState } from "../../shared/provider-runtime-domain";
+
+export interface ProviderBackgroundCreateResult {
+  responseId: string;
+  status: string;
+  rawResponseJson: string;
+  rateLimitState: ProviderObservedRateLimitState | null;
+}
+
+export interface ProviderBackgroundRetrieveResult {
+  responseId: string;
+  status: string;
+  text: string | null;
+  rawResponseJson: string;
+  rateLimitState: ProviderObservedRateLimitState | null;
+}
+
+export function backgroundUnsupported(providerLabel: string): () => Promise<ProviderBackgroundCreateResult> {
+  return async () => {
+    throw new Error(`${providerLabel} background execution is unavailable.`);
+  };
+}
+
+export function retrieveUnsupported(providerLabel: string): () => Promise<ProviderBackgroundRetrieveResult> {
+  return async () => {
+    throw new Error(`${providerLabel} response retrieval is unavailable.`);
+  };
+}