DEVX-48: fix: use heredoc syntax for multi-line $GITHUB_ENV values

tests/unit/test_distribute_files.py

@@ -101,6 +101,23 @@ class TestCli:
         assert "ASSIGNED_FILES=" in content
         assert "SKIP=false" in content
 
+    def test_github_env_multiline_uses_heredoc(self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+        gh_file = tmp_path / "env.txt"
+        monkeypatch.setenv("GITHUB_ENV", str(gh_file))
+        for i in range(6):
+            (tmp_path / f"test_{i}.py").write_text("")
+        runner = CliRunner()
+        result = runner.invoke(
+            main,
+            ["--pattern", str(tmp_path / "test_*.py"), "--runner-index", "1", "--max-runners", "2", "--github-env"],
+        )
+        assert result.exit_code == 0
+        content = gh_file.read_text()
+        # Multi-line values must use heredoc syntax to avoid corrupting $GITHUB_ENV
+        assert "ASSIGNED_FILES<<EOF" in content
+        assert content.count("EOF") >= 2
+        assert "SKIP=false" in content
+
     def test_skip_if_excess(self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
         gh_file = tmp_path / "env.txt"
         monkeypatch.setenv("GITHUB_ENV", str(gh_file))