This PR's goal is to improve discoverability of how Zed "remembers" the
currently selected agent when hitting `cmd-n` (or `ctrl-n`). Hitting
that binding starts a new thread with whatever agent is currently
selected.

In the example below, I am in a Claude Code thread and if I hit `cmd-n`,
a new, fresh CC thread will be started:

<img width="500" height="822" alt="Screenshot 2025-11-18 at 1  13@2x"
src="https://github.com/user-attachments/assets/d3acd1aa-459d-4078-9b62-bbac3b8c1600"
/>


Release Notes:

- agent: Improved discoverability of the `cmd-n` keybinding to create a
new thread with the currently selected agent.