agent_ui: Show the "new thread" keybinding for the currently active agent (#42939)

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.

assets/keymaps/default-macos.json

@@ -313,7 +313,7 @@
     "use_key_equivalents": true,
     "bindings": {
       "cmd-n": "agent::NewTextThread",
-      "cmd-alt-t": "agent::NewThread"
+      "cmd-alt-n": "agent::NewExternalAgentThread"
     }
   },
   {