  The $^{30}$P$(p,\gamma)^{31}$S reaction plays an important role in
understanding nucleosynthesis of $A\geq 30$ nuclides in oxygen-neon novae. The
Gaseous Detector with Germanium Tagging was used to measure $^{31}$Cl
$\beta$-delayed proton decay through the key $J^{\pi}=3/2^{+}$, 260-keV
resonance. The intensity $I^{260}_{\beta p} = 8.3^{+1.2}_{-0.9} \times 10^{-6}$
represents the weakest $\beta$-delayed, charged-particle emission ever measured
below 400 keV, resulting in a proton branching ratio of $\Gamma_p / \Gamma =
2.5^{+0.4}_{-0.3} \times 10^{-4}$. By combining this measurement with
shell-model calculations for $\Gamma_{\gamma}$ and past work on other
resonances, the total $^{30}$P$(p,\gamma)^{31}$S rate has been determined with
reduced uncertainty. The new rate has been used in hydrodynamic simulations to
model the composition of nova ejecta, leading to a concrete prediction of
$^{30}$Si/$^{28}$Si excesses in presolar nova grains and the calibration of
nuclear thermometers.
