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/envelope.py

@@ -0,0 +1,53 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+"""BEHAVE-TEXT Observation envelope (registry-aware subclass).
+
+Mirrors BEHAVE-SHELL's pattern: structural envelope from `behave-core`,
+registry-aware validation added here against BEHAVE-TEXT's `PRIMITIVE_REGISTRY`.
+
+PII discipline (TIGHTER for text than for shell):
+  text-domain sensors operate on raw message bodies. They MUST hash, aggregate,
+  or categorize before constructing an Observation — never put message text
+  into the `value` or `evidence_ref` field. `evidence_ref` should point at an
+  external message-store record (e.g. a Telegram message ID), not at the text.
+"""
+
+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):
+    """Text-domain Observation: base envelope + BEHAVE-TEXT registry check."""
+
+    @model_validator(mode="after")
+    def _validate_against_text_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",
+]