How can one tell from the MO diagram of the cyanide ion that the HOMO is carbon-centred?
The 2s and 2pz atomic orbitals are of the same symmetry in the molecule. That means that the 2 sigma* and 3 sigma are actually hybrids. The former is a hybrid were most of the electron density points to the bond (making it partially bonding), while in the latter most of the electron density points outside, i.e. a lone pair in the carbon.