FR: spy on a session

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...

Start by tracing how shpool represents terminal state and generates output during session resume. Define the smallest read-only entry point that exposes the current snapshot without resuming or attaching, including behavior while another client is attached. Done means a command can retrieve the session's current rendered state without changing the session.