FR: spy on a session

[skissane-medallia]

AI Policy Ack
Ack

What new feature do you want?
Since shpool keeps an in-memory representation of the terminal state, it would be great if there were a command to "spy" on the session – to get the same output shpool would generate at session resume, without actually resuming the session.

This is similar to #40 but is a smaller ask (read-only snapshot of current state). Of course, if #40 was implemented, this could potentially be done via read-only attach then detach again as soon as the initial re-render is finished. But possibly this feature could be implemented without without doing #40, and with significantly less work than #40 would require.

Use case: I have a web app which shows running sessions and let's me launch a new one. (It doesn't attach to them, I need to use a terminal emulator for that.) It would be great if I could "peek" from the web app at the current state of the session (say it is running some long-running AI agent, I want to see what the agent is doing). Maybe I'm already attached to the session from a terminal emulator on my laptop sitting on my desk, but I'm checking the webapp from my phone...

[ethanpailes]

This seems like a reasonable feature. Implementing it would probably be pretty simillar to simultanious connections. I've rejiggered the architecture of the shpool daemon since writing #40 so it should now be easier to accomplish. This would just be a special case of simultanious connections where we hang up immediately uppon session restore, so really there would be three different types of simultanious connections: a full readwrite connection, a readonly connection (basically just tail -f), and a readonly connection with immediate hangup (this FR).