Solution (source code)

= Solution

The displayed basis obeys $[\tau_a,\tau_b]=\epsilon_{abc}\tau_c$. Therefore
$$
\kappa(\tau_a,\tau_b)
=\epsilon_{ac d}\epsilon_{bd c}=-2\delta_{ab}.
$$
The basis is adapted to the Killing form in the usual orthogonal-basis sense, since its Gram matrix is diagonal; rescaling by $1/\sqrt2$ makes it orthonormal for $-\kappa$.