Relicense under GPL-3.0-or-later

Replace MIT with the full GNU GPLv3 text, update license metadata in
pyproject.toml (+ trove classifiers) and PKGBUILD, and add
SPDX-License-Identifier headers to all Python modules and shell scripts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/sentinel.sh

@@ -1,4 +1,5 @@
 #!/bin/bash
+# SPDX-License-Identifier: GPL-3.0-or-later
 # Enodia Sentinel — a host intrusion-detection daemon.
 #
 # Continuously runs a set of detectors over live system state (processes,