Make derived AI rules more accurately reflect chat reversions / rollbacks

[jakelevirne]

From Talib on the community Slack:

Hey I am trying out SpecStory and am trying out the experimental derived rules feature. One thing I notice is that it doesn't remove anything that was added to the mdc file. During the development of a project I often go back and revert to a previous edit from the chat (or I may not revert and instead change code from a new prompt), in either case the derived rules shouldn't reference any changes that I didn't keep (or that I accepted but changed). It's almost like it's logging changes instead of defining rules.