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/behave_shell/spec/__init__.py

@@ -0,0 +1,37 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""BEHAVE observation envelope and primitive registry — DECNET-aligned.
+
+Public API:
+
+    from spec import Observation, Window, OBSERVATION_SCHEMA_VERSION
+    from spec import PRIMITIVE_REGISTRY, ValueKind, ValueTypeSpec
+    from spec import event_topic_for, to_event_payload, from_event_payload
+
+See ``spec.envelope`` for the central PII-discipline statement that binds every
+sensor emitting BEHAVE observations.
+"""
+
+from .envelope import OBSERVATION_SCHEMA_VERSION, Observation, ObservationValue, Window
+from .event_adapter import (
+    TOPIC_PREFIX,
+    event_topic_for,
+    from_event_payload,
+    to_event_payload,
+)
+from .primitives import PRIMITIVE_REGISTRY, ValueKind, ValueTypeSpec, get, is_known
+
+__all__ = [
+    "OBSERVATION_SCHEMA_VERSION",
+    "Observation",
+    "ObservationValue",
+    "Window",
+    "PRIMITIVE_REGISTRY",
+    "ValueKind",
+    "ValueTypeSpec",
+    "is_known",
+    "get",
+    "TOPIC_PREFIX",
+    "event_topic_for",
+    "to_event_payload",
+    "from_event_payload",
+]