Update assistant to agent in settings and keymaps (#29943)

Closes #ISSUE

Release Notes:

- Agent Beta: Renamed the top-level `assistant` settings key to `agent`.
A migration for existing settings files is included.
- Agent Beta: Moved the `assistant::ToggleFocus`,
`assistant::ToggleModelSelector`, and `assistant::OpenRulesLibrary`
actions to the `agent` namespace. Existing keymaps that mention these
actions by their old names will continue to work.

---------

Co-authored-by: Max <max@zed.dev>

crates/vim/src/command.rs

@@ -876,7 +876,7 @@ fn generate_commands(_: &App) -> Vec<VimCommand> {
         VimCommand::str(("C", "ollab"), "collab_panel::ToggleFocus"),
         VimCommand::str(("Ch", "at"), "chat_panel::ToggleFocus"),
         VimCommand::str(("No", "tifications"), "notification_panel::ToggleFocus"),
-        VimCommand::str(("A", "I"), "assistant::ToggleFocus"),
+        VimCommand::str(("A", "I"), "agent::ToggleFocus"),
         VimCommand::str(("G", "it"), "git_panel::ToggleFocus"),
         VimCommand::new(("noh", "lsearch"), search::buffer_search::Dismiss),
         VimCommand::new(("$", ""), EndOfDocument),