reseed_if_terminal created its temp as <auth>.rebootstrap.<pid>.tmp with
O_CREAT|O_EXCL. Boot-hook PIDs inside a container are near-deterministic,
so a run SIGKILL'd between create and replace leaves a same-named file and
every later boot hits FileExistsError - which main() swallows as
"error (ignored)", leaving the terminal-session recovery path dead until
someone deletes the temp by hand.
tempfile.mkstemp in the auth dir gives a random name at 0600 (stdlib only,
matching the script's no-hermes-imports rule); the fsync + os.replace +
unlink-on-failure semantics are unchanged.
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