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.

docs/guide/tools/demo-shell.sh

@@ -1,4 +1,6 @@
 #!/bin/bash
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright (C) 2026 Andy Dixon
 # ForceCommand for the throwaway screenshot sshd: never a login shell (no ~/.profile, no byobu).
 DEMO_HOME=$1 SFTP_SERVER=$2
 export LC_ALL=C.UTF-8

docs/guide/tools/fakes.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright (C) 2026 Andy Dixon
 """Tiny fake servers for the user-guide screenshots: RAW sensor feed (42023), IRC (42667),
 GitHub API (42080) and a VNC desktop (42900). Stdlib only. Runs until killed."""
 

docs/guide/tools/screenshots.sh

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright (C) 2026 Andy Dixon
 # Regenerates docs/guide/img/*.png: a throwaway sshd (ForceCommand, never a login shell),
 # fake RAW/IRC/GitHub/VNC servers, a demo data directory, then one headless app run per shot.
 # Needs: xvfb-run, sshd, ssh-keygen, python3, ImageMagick. Usage: docs/guide/tools/screenshots.sh [shot...]