docs(core): add 0.8.0 release notes for Linker.backend change

Document the breaking change from property to classmethod. Also
remove unused pytest import from test_linker_backend.py.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cpcloud

cuda_core/docs/source/release/0.8.0-notes.rst

@@ -0,0 +1,17 @@
+.. SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+.. SPDX-License-Identifier: Apache-2.0
+
+.. currentmodule:: cuda.core
+
+``cuda.core`` 0.8.0 Release Notes
+=================================
+
+
+Breaking Changes
+----------------
+
+- :meth:`Linker.backend` is now a classmethod instead of an instance property.
+  Call sites must use ``Linker.backend()`` (with parentheses) instead of
+  ``linker.backend``. This allows querying the linking backend without
+  constructing a ``Linker`` instance — for example, to choose between PTX and
+  LTOIR input before linking.

cuda_core/tests/test_linker_backend.py

@@ -11,8 +11,6 @@
 
 import inspect
 
-import pytest
-
 import cuda.core._linker as _linker
 from cuda.core._linker import Linker