Closes
https://linear.app/zed-industries/issue/Z-1890/save-assistant-conversations-to-the-filesystem
Closes
https://linear.app/zed-industries/issue/Z-2459/cycling-message-roles-on-the-last-empty-message-alters-the-message
Closes
https://linear.app/zed-industries/issue/Z-2460/cycling-role-in-an-empty-message-cycles-wrong-messages-role
Closes https://linear.app/zed-industries/issue/Z-2365/assistant-toolbar
Closes
https://linear.app/zed-industries/issue/Z-2461/always-insert-an-empty-message-at-the-end-of-the-conversation

Release Notes:

- You can now save conversations with the assistant to
`~/.config/zed/conversations` with `cmd-s`. Conversations are also
automatically saved as they are edited.