license: release Nexus Terminal under the GNU AGPL v3

- LICENSE: the GNU Affero General Public License, version 3 (verbatim
  from gnu.org).
- SPDX-License-Identifier: AGPL-3.0-only plus a copyright line on every
  first-party source file (Rust, shell, Python, PHP); vendored crates keep
  their own licenses.
- Cargo manifests carry license/authors/repository; the macOS bundle
  copyright and the About page state the license and link the source.
- Release zips (and the macOS app bundle) include LICENSE.
- README: License section.

updates/index.php

@@ -1,4 +1,6 @@
 <?php
+// SPDX-License-Identifier: AGPL-3.0-only
+// Copyright (C) 2026 Andy Dixon
 // Query parameters may arrive as arrays (?arch[]=x); treat anything but a string as absent.
 $param = function ($name) { return isset($_GET[$name]) && is_string($_GET[$name]) ? $_GET[$name] : ''; };
 $platform = $param('platform');