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/oauth/oauth-page.ts

@@ -0,0 +1,49 @@
+/** Minimal HTML shown in the user's browser after the OAuth redirect. */
+
+function escapeHtml(value: string): string {
+  return value
+    .replaceAll("&", "&")
+    .replaceAll("<", "&lt;")
+    .replaceAll(">", "&gt;")
+    .replaceAll('"', "&quot;")
+    .replaceAll("'", "&#39;");
+}
+
+function renderPage(heading: string, message: string, details?: string): string {
+  return `<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1" />
+  <title>${escapeHtml(heading)}</title>
+  <style>
+    * { box-sizing: border-box; }
+    html { color-scheme: dark; }
+    body {
+      margin: 0; min-height: 100vh; display: flex; align-items: center; justify-content: center;
+      padding: 24px; background: #09090b; color: #fafafa; text-align: center;
+      font-family: ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
+    }
+    main { width: 100%; max-width: 520px; }
+    h1 { margin: 0 0 10px; font-size: 26px; font-weight: 650; }
+    p { margin: 0; line-height: 1.7; color: #a1a1aa; font-size: 15px; }
+    .details { margin-top: 14px; font-family: ui-monospace, Menlo, Consolas, monospace; font-size: 13px; color: #a1a1aa; word-break: break-word; }
+  </style>
+</head>
+<body>
+  <main>
+    <h1>${escapeHtml(heading)}</h1>
+    <p>${escapeHtml(message)}</p>
+    ${details ? `<div class="details">${escapeHtml(details)}</div>` : ""}
+  </main>
+</body>
+</html>`;
+}
+
+export function oauthSuccessHtml(message: string): string {
+  return renderPage("Authentication successful", message);
+}
+
+export function oauthErrorHtml(message: string, details?: string): string {
+  return renderPage("Authentication failed", message, details);
+}