Make write_executive_report atomic to avoid destroying prior report

write_executive_report opened the destination with path.open("w"),
which truncates the file the moment it's opened, before any content
is written. Every other writer in this module already goes through
_atomic_write_text, which writes to a temp file and renames it into
place. Align write_executive_report with the same pattern so a failed
write can no longer destroy the previous penetration_test_report.md.

Closes #828.

strix/report/writer.py

@@ -115,10 +115,12 @@ def write_run_record(run_dir: Path, run_record: dict[str, Any]) -> None:
 
 def write_executive_report(run_dir: Path, final_scan_result: str) -> None:
     path = run_dir / "penetration_test_report.md"
-    with path.open("w", encoding="utf-8") as f:
-        f.write("# Security Penetration Test Report\n\n")
-        f.write(f"**Generated:** {datetime.now(UTC).strftime('%Y-%m-%d %H:%M:%S UTC')}\n\n")
-        f.write(f"{final_scan_result}\n")
+    content = (
+        "# Security Penetration Test Report\n\n"
+        f"**Generated:** {datetime.now(UTC).strftime('%Y-%m-%d %H:%M:%S UTC')}\n\n"
+        f"{final_scan_result}\n"
+    )
+    _atomic_write_text(path, content)
     logger.info("Saved final penetration test report to: %s", path)
 
 

tests/test_report_writer.py

@@ -179,3 +179,21 @@ def test_write_executive_report_writes_markdown(tmp_path: Path) -> None:
     content = (tmp_path / "penetration_test_report.md").read_text(encoding="utf-8")
     assert "# Security Penetration Test Report" in content
     assert "Scan complete. No critical issues." in content
+
+
+def test_write_executive_report_preserves_previous_report_on_failure(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    report_path = tmp_path / "penetration_test_report.md"
+    report_path.write_text("previous report", encoding="utf-8")
+
+    def boom(*_args: Any, **_kwargs: Any) -> None:
+        raise OSError("disk full")
+
+    monkeypatch.setattr("strix.report.writer.tempfile.NamedTemporaryFile", boom)
+
+    with pytest.raises(OSError, match="disk full"):
+        write_executive_report(tmp_path, "new content")
+
+    assert report_path.read_text(encoding="utf-8") == "previous report"