No-op ALLM.Pipeline.Lock — the current default.
Runs the pipeline directly with no cross-run serialization and, crucially,
without a Repo.checkout/2 pinning a connection for the run's duration (the
failure mode described in ALLM.Pipeline.Lock). Two concurrent
runs of the same pipeline are therefore possible again; if that becomes a
real problem, restore a connection-free guard rather than
ALLM.Pipeline.Lock.Advisory.