skip tasks if inputs are unchanged?

[amonks]

this sort of feature is a staple of this sort of task running system, but adds a lot of user-facing complexity

• we'd need some persistent state between runs
• tasks would need to specify the files they depend on
• we'd need to dirhash those files and put them in the state
• we'd need to skip tasks if the hash hadn't changed

I feel like there's more complexity to this, but those steps are the obvious ones off the top of my head

just adding persistent state is a pretty significant change (not, like, difficult, but significant)