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/language_model_selector/src/language_model_selector.rs

@@ -15,9 +15,12 @@ use proto::Plan;
 use ui::{ListItem, ListItemSpacing, PopoverMenu, PopoverMenuHandle, PopoverTrigger, prelude::*};
 
 action_with_deprecated_aliases!(
-    assistant,
+    agent,
     ToggleModelSelector,
-    ["assistant2::ToggleModelSelector"]
+    [
+        "assistant::ToggleModelSelector",
+        "assistant2::ToggleModelSelector"
+    ]
 );
 
 const TRY_ZED_PRO_URL: &str = "https://zed.dev/pro";