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/macos/build-app.sh

@@ -1,4 +1,6 @@
 #!/bin/sh
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright (C) 2026 Andy Dixon
 # Build NexusTerminal.app, a macOS application bundle. Run on a Mac with Xcode command-line tools:
 #
 #   rust/packaging/macos/build-app.sh            # universal (Apple Silicon + Intel) if both
@@ -44,6 +46,7 @@ mkdir -p "$APP/Contents/MacOS" "$APP/Contents/Resources"
 cp "$BIN" "$APP/Contents/MacOS/NexusTerminal"
 chmod +x "$APP/Contents/MacOS/NexusTerminal"
 cp "$HERE/NexusTerminal.icns" "$APP/Contents/Resources/NexusTerminal.icns"
+cp "$HERE/../../../LICENSE" "$APP/Contents/Resources/LICENSE"
 sed "s/__VERSION__/$VERSION/g" "$HERE/Info.plist" > "$APP/Contents/Info.plist"
 printf 'APPL????' > "$APP/Contents/PkgInfo"