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.

README.md

@@ -67,3 +67,17 @@ cargo run --release
 ```
 
 See [docs/build-and-release.md](docs/build-and-release.md) for platform prerequisites, tests and release packaging.
+
+## License
+
+Copyright (C) 2026 Andy Dixon.
+
+Nexus Terminal is free software: you can redistribute it and/or modify it under the terms of the
+[GNU Affero General Public License, version 3](LICENSE) as published by the Free Software
+Foundation. It is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+license for details.
+
+Third-party crates (including the patched copies in `rust/vendor/`) keep their own licenses,
+which are listed in each crate.
+