This may be a little basic, but if you're already going with the "efficient at transmitting forces" idea for physical posture, then I think a good analogy in terms of mental posture would simply be "efficient at processing information" (of which being rational is a pretty useful method, just like keeping your spine, er, non-kyphotic is useful).

This is much more concise than:

how efficiently one's patterns of directing attention let one mentally navigate one's environment

while at the same time relatively neutral with respect to the actual goals that you may have (e.g. keeping one's balance, either physically when someone bumps into you, or mentally when someone offends you).

And if you were to compare mental posture to an artform like aikido on the physical side, you might also get something like the noble eightfold path on the mental side (which, sayings like having "right mindfulness" can be compared with "relax more").

Also, with the amount of upvotes and relevance to this topic, I'm surprised to see that Roles are Martial Arts for Agency hasn't been linked yet. It was definitely in the back of my mind as I made a few comparisons.