Allow the Thread Switcher to stack above the Agent Conversation

[zambetti]

Opening a new discussion here as suggested by @macraig in #55157. See that discussion for more detail.

With Zed as a full-height window, the sidebar ends up being either mostly empty space or old content, so it is often desirable to toggle it open and closed. If it were possible to stack the Thread Switcher over the Agent Conversation, we could make better use of that vertical space and reclaim some crucial horizontal room.

@domindel built a great working proof-of-concept fork (domindel/zed-55157) that demonstrates how this feels. It even supports vertical resizing.

A quick before-and-after mockup...