Follow up to https://github.com/zed-industries/zed/pull/53574
Closes https://github.com/zed-industries/zed/issues/53705

In the draft feature PR, we didn't update the new thread creation from
the agent panel's toolbar to _also_ create a corresponding draft item in
the sidebar. Additionally, the draft feature exposed a
previously-existing bug where the toolbar wouldn't get synced with the
currently selected agent; and as you navigated between them, we would
fall back to the native agent, as well as when you hit cmd-n.

cc @mikayla-maki just a quick FYI here for the revamp! Will push this
forward in the meantime given it's a bit of a bad bug.

Release Notes:

- N/A