A 0% batteryLevel is an invalid/no-data value (no battery wired or
misconfigured node), not a genuinely critical battery, so it must not
produce a "⚠️ battery low" alert. Add a non-positive guard to
shouldAlert after the undefined check: 0 (and any non-positive) now
skips, while 1-20 still alert and >20 still doesn't. Threshold/cooldown/
batching unchanged.
Tests: 3 new specs in the check-threshold block (0% excluded from the
batch, lone 0% sends nothing, 1% still alerts). 180 -> 183.