Read back all suite summaries the workers wrote during this run, sorted by
suite name for stable output.
Parameters
sinceMs: {sinceMs?:number} = {}
When provided, only artifacts created at or after this
timestamp are returned, so summaries from a previous run that share the
report directory are ignored. A one-second slack is allowed to absorb
small clock differences between worker processes.
Read back all suite summaries the workers wrote during this run, sorted by suite name for stable output.