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-oauth-service.ts

@@ -0,0 +1,40 @@
+import { getOAuthFlow } from "./oauth/oauth-flows";
+
+export interface ProviderOAuthHooks {
+  onAuth: (info: { url: string; instructions?: string }) => void;
+  onProgress?: (message: string) => void;
+}
+
+/**
+ * Drives a provider's native OAuth login and returns the obtained credentials
+ * as a JSON string, ready to be stored through the existing secret vault. The
+ * service is decoupled from Electron: opening the browser and emitting UI
+ * events are supplied by the caller via {@link ProviderOAuthHooks}.
+ */
+export class ProviderOAuthService {
+  private readonly controllers = new Map<string, AbortController>();
+
+  async run(input: { providerKey: string; enterpriseDomain?: string | null }, hooks: ProviderOAuthHooks): Promise<string> {
+    const flow = getOAuthFlow(input.providerKey);
+    if (this.controllers.has(input.providerKey)) {
+      throw new Error("A sign-in for this provider is already in progress.");
+    }
+    const controller = new AbortController();
+    this.controllers.set(input.providerKey, controller);
+    try {
+      const credentials = await flow.login({
+        onAuth: hooks.onAuth,
+        onProgress: hooks.onProgress,
+        signal: controller.signal,
+        enterpriseDomain: input.enterpriseDomain ?? null,
+      });
+      return JSON.stringify(credentials);
+    } finally {
+      this.controllers.delete(input.providerKey);
+    }
+  }
+
+  cancel(providerKey: string): void {
+    this.controllers.get(providerKey)?.abort();
+  }
+}