What might you imagine being different in an “agent first OS” compared to a terminal only Linux distribution?

I can imagine a different UI. Instead of a browser with tabs and an address bar, you have a series of infinitely scrolling "terminals" with a chat bar at the bottom. Each one is just a chat session into which objects like web pages, images, etc can be embedded and they scroll horizontally a bit like multiple desktops do. You could zoom in and out as much as you want, the goal is to let you easily monitor many parallel tasks.

From the AI's perspective a filesystem that vector indexes data on the fly would make sense, perhaps, along with an ability for the user to share out fine-grained permissions with it.

No I/O, ability to mount vectors and graphs, bare metal file system, structured traces, deterministic replayability of commands