refactor: remove molecule_ci_guard — adds complexity, no value

The cross-runner cancellation guard was killing healthy molecule slots
when unhealthy slots failed, defeating fail-fast: false. With fail-fast:
false and no guard, slots on healthy runners complete independently.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_cli.py

@@ -40,7 +40,6 @@ class TestCliGroups:
         result = runner.invoke(cli, ["molecule", "--help"])
         assert result.exit_code == 0
         assert "distribute" in result.output
-        assert "guard" in result.output
         assert "all" in result.output
 
 
@@ -231,13 +230,6 @@ class TestMoleculeCommands:
         assert result.exit_code == 0
         mock_run.assert_called_once_with("devx.molecule.discover_runners", [])
 
-    @patch("devx.cli._run_module")
-    def test_molecule_guard(self, mock_run: MagicMock) -> None:
-        runner = CliRunner()
-        result = runner.invoke(cli, ["molecule", "guard"])
-        assert result.exit_code == 0
-        mock_run.assert_called_once_with("devx.molecule.molecule_ci_guard", [])
-
     @patch("devx.cli._run_module")
     def test_molecule_all(self, mock_run: MagicMock) -> None:
         runner = CliRunner()