Document copy button setup and add tests

tests/test_docs_copybutton.py

@@ -0,0 +1,17 @@
+from pathlib import Path
+
+
+def test_docs_dependency_includes_copybutton() -> None:
+    """Docs dependency group must install sphinx-copybutton."""
+
+    text = Path("pyproject.toml").read_text(encoding="utf-8")
+    assert "sphinx-copybutton" in text
+
+
+def test_docs_conf_enables_copybutton() -> None:
+    """Sphinx config must enable the extension and prompt stripping."""
+
+    conf = Path("docs/conf.py").read_text(encoding="utf-8")
+    assert '"sphinx_copybutton"' in conf
+    assert "copybutton_prompt_text" in conf
+