fix(export): replace JSONL targets atomically

[samzong]

Summary

• write session JSONL exports to a same-directory temporary file
• sync and atomically replace the output only after export succeeds
• preserve an existing target and remove temporary output when export fails

Closes release blocker NG-08.

Verification

• cargo test session::tests::jsonl_file_export_replaces_target_only_after_success -- --exact
• make check

[samzong]

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