refactor: drop decnet- prefix — BEHAVE is now standalone

Rename packages and imports:
  decnet-behave-core  → behave-core
  decnet-behave-shell → behave-shell
  decnet-behave-text  → behave-text
  decnet_behave_*     → behave_*

BEHAVE is no longer a DECNET sub-project.

BEHAVE-SHELL/tests/test_event_adapter.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 
 import pytest
 
-from decnet_behave_shell.spec import (
+from behave_shell.spec import (
     Observation,
     Window,
     event_topic_for,