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-TEXT/behave_text/spec/__init__.py

@@ -0,0 +1,43 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""BEHAVE-TEXT spec — text/messaging-domain registry, layered on behave-core.
+
+Public API:
+
+    from spec import Observation, Window, OBSERVATION_SCHEMA_VERSION
+    from spec import PRIMITIVE_REGISTRY, ValueKind, ValueTypeSpec
+    from spec import TOPIC_PREFIX, event_topic_for
+
+The ``Observation`` exported here is a registry-aware subclass of the base
+class from ``behave-core``; it validates that ``primitive`` is in the
+text registry and that ``value`` matches the registry's per-primitive spec.
+
+See ``spec.envelope`` (and the core envelope module) for PII discipline.
+"""
+
+from .envelope import OBSERVATION_SCHEMA_VERSION, Observation, ObservationValue, Window
+from .primitives import PRIMITIVE_REGISTRY, ValueKind, ValueTypeSpec, get, is_known
+
+# Topic namespace deliberately uses *actor* (not *attacker*) because chat-group
+# members may include observers, brokers, victims, and bystanders alongside
+# threat actors. Attribution of role is the engine's job, not BEHAVE-TEXT's.
+TOPIC_PREFIX: str = "actor.observation.text"
+
+
+def event_topic_for(primitive: str) -> str:
+    """Return the canonical bus topic for a BEHAVE-TEXT primitive."""
+    return f"{TOPIC_PREFIX}.{primitive}"
+
+
+__all__ = [
+    "OBSERVATION_SCHEMA_VERSION",
+    "Observation",
+    "ObservationValue",
+    "Window",
+    "PRIMITIVE_REGISTRY",
+    "ValueKind",
+    "ValueTypeSpec",
+    "is_known",
+    "get",
+    "TOPIC_PREFIX",
+    "event_topic_for",
+]