* fix(subagents): report an explicit zero batch limit instead of defaulting it
SubagentBatchService.submit resolved max_live_items / max_running_items with
`or`, so a caller that explicitly passed 0 got the configured default
(100 / 3) persisted and the 1..N range guards never saw the value, while a
negative already failed there. Resolve the defaults on 'is None' so an
explicit 0 reaches the guard that names it.
* docs(subagents): state the >= 1 window constraint in the batch_task args
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>