Solution (source code)

= Solution

For the lowest <eigenvalue>, $-1$, solving $(H+I)\mathbf v=0$ gives $v_2=0$ and $v_3=-v_1$. A unit <eigenvector> is therefore
$$
\boxed{\mathbf v=\frac1{\sqrt2}(1,0,-1)^T.}
$$
Any nonzero <scalar multiple> is an equally valid <eigenvector>.