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/envelope.py

@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""BEHAVE-SHELL Observation envelope (registry-aware subclass).
+
+The base envelope (`Observation`, `Window`, `OBSERVATION_SCHEMA_VERSION`,
+`ObservationValue`) lives in `behave-core`; it enforces only structural
+invariants (window ordering, confidence bounds, schema version, no extras).
+
+This module subclasses the core `Observation` to add registry-aware validation
+against `BEHAVE-SHELL`'s `PRIMITIVE_REGISTRY`. The subclass is exported under
+the same name `Observation` so existing imports (``from spec.envelope import
+Observation``) continue to resolve to the registry-validated form without
+consumer changes.
+
+PII discipline (lifted from DECNET ``attackers.py:268-285,308-311``) — see the
+core envelope module docstring for the binding statement.
+"""
+
+from __future__ import annotations
+
+from pydantic import model_validator
+
+from behave_core.spec.envelope import (
+    OBSERVATION_SCHEMA_VERSION,
+    ObservationValue,
+    Window,
+)
+from behave_core.spec.envelope import Observation as _BaseObservation
+
+from .primitives import PRIMITIVE_REGISTRY
+
+
+class Observation(_BaseObservation):
+    """Shell-domain Observation: base envelope + BEHAVE-SHELL registry check."""
+
+    @model_validator(mode="after")
+    def _validate_against_shell_registry(self) -> "Observation":
+        spec = PRIMITIVE_REGISTRY.get(self.primitive)
+        if spec is None:
+            raise ValueError(
+                f"unknown primitive {self.primitive!r}; "
+                f"add it to spec/primitives.py:PRIMITIVE_REGISTRY first"
+            )
+        try:
+            spec.validate_value(self.value)
+        except ValueError as exc:
+            raise ValueError(
+                f"value invalid for primitive {self.primitive!r}: {exc}"
+            ) from None
+        return self
+
+
+__all__ = [
+    "OBSERVATION_SCHEMA_VERSION",
+    "Observation",
+    "ObservationValue",
+    "Window",
+]