docs: define strict dry-run contract

# Conflicts:
#	.secrets.baseline

tests/e2e/test_cli.py

@@ -11,6 +11,23 @@ from pronote_sync.errors import PipelineCriticalError, PipelineWarning
 from pronote_sync.models.pronote import PronoteData
 
 
+def test_help_documents_strict_dry_run_contract(capsys: pytest.CaptureFixture[str]) -> None:
+    """L'aide CLI expose le contrat strict et l'incompatibilité QR/token.
+
+    :param capsys: Capture des sorties standard de pytest.
+    :return: None
+    """
+    from pronote_sync.cli.main import main
+
+    with pytest.raises(SystemExit) as exc_info:
+        main(["--help"])
+
+    output = " ".join(capsys.readouterr().out.split())
+    assert exc_info.value.code == 0
+    assert "sans sortie distante ni état local persistant" in output
+    assert "PRONOTE_AUTH_MODE=qr_token" in output
+
+
 def test_main_runs_composition_root_in_dry_run_with_requested_log_level(
     mocker: MockerFixture,
 ) -> None: