Prior art in GMail is they use j k for this, clearly we can not use these as they are already taken. But it is very interesting they picked to use the same shortcut they use in the list.
I am trying to comb through existing shortcuts to see if we can do a single letter here that is intuitive. arrow right and left are intuitive, but missing from some keyboards and may be too easy to mis-press, I will start I guess with a 2 letter shortcut unless I get a magical brainwave.