Traceability files are not written atomically

[JeromeMaag]

export_summary.txt and manifest.json are currently not written atomically.

If writing manifest.json fails after export_summary.txt was already written, the export run is internally marked as Failed, but the existing summary can still show Export status: Completed.

Reproduced by forcing chat_export.export_summary.json.dump to fail:

• internal result status: Failed
• manifest.json exists but has size 0
• export_summary.txt still exists and reports Export status: Completed

This creates inconsistent traceability artifacts for the same export run.