The live pair was replaced with two renames, key then cert. A run
stopped between them (timeout, container stop, power loss) left a new
key beside the old cert, and every later run logged in with that pair,
so the agent could not re-pull until its container restarted.
Each pair now goes into a fresh directory under
/run/hive-agent-bao-identity, and `current`, a symlink to it, is
replaced by one rename. Wherever a run stops, `current` names one whole
pair; the next run removes whatever a stopped run left. Consumers read
`current/cert` and `current/key` through the same read-only options and
still re-read them on every login.
The script test covers a swap stopped at each of its link and rename
steps: the live pair is always one whole pair, and the next run puts
the new one live.
Refs #5017
4c4ecb8ed9