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_primitives.py

@@ -11,7 +11,7 @@ from __future__ import annotations
 import re
 from pathlib import Path
 
-from decnet_behave_shell.spec import PRIMITIVE_REGISTRY, ValueKind
+from behave_shell.spec import PRIMITIVE_REGISTRY, ValueKind
 
 # Primitive paths expected by scratchpad.md (hand-extracted; v0.1).
 EXPECTED_PRIMITIVES = {