Expect to be able to use `/env` in new chat sessions

[bamurtaugh]

Testing #323402

I can use /env in existing chat sessions in the editor and Agents windows. However, I would've expected to be able to use it in new sessions too, so that I can check my environment is what I'd expect before kicking off a new agent.

[DonJayamanne]

This is interesting, the problem is we haven't started a session yet, hence we cannot get completions from the SDK.
I believe this is a bug, as we cannot even run commands like /init /fleet /init or the like.
Will need to find a better way to get these working or hardcode some of these?