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.

rust/packaging/linux/install.sh

@@ -1,4 +1,6 @@
 #!/bin/sh
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright (C) 2026 Andy Dixon
 # Install Nexus Terminal for the current user so it appears in the app launcher.
 # Usage: rust/packaging/linux/install.sh [path/to/nexus-terminal]   (default: target/release/nexus-terminal)
 set -eu