One-sided or two-sided, and when halving is fair

A strategy's mean daily return is tested against zero and yields $z = 1.8$. Before running the backtest, the researcher had committed to a directional hypothesis: the strategy makes money (mean return $> 0$).

Compute the one-sided p-value for $z = 1.8$, state how it compares to the two-sided p-value, and explain the rule for when you are allowed to use the one-sided version.