Solution (source code)

= Solution
{c}

The <ground state> is the single <energy eigenstate> $\psi_0$. Its coefficient in the normalized <wavefunction> is $\sqrt{2/3}$, so the <Born rule> gives
$$
\boxed{\Pr(E=0)=\left|\sqrt{\frac23}\right|^2=\frac23.}
$$
This <probability> is independent of $t$ because <unitary time evolution> changes only the <dynamical phases> of the <energy eigenstates>.