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/migrator/src/patterns.rs

@@ -7,6 +7,7 @@ pub(crate) use keymap::{
 };
 
 pub(crate) use settings::{
-    SETTINGS_ASSISTANT_TOOLS_PATTERN, SETTINGS_LANGUAGES_PATTERN,
+    SETTINGS_ASSISTANT_PATTERN, SETTINGS_ASSISTANT_TOOLS_PATTERN,
+    SETTINGS_EDIT_PREDICTIONS_ASSISTANT_PATTERN, SETTINGS_LANGUAGES_PATTERN,
     SETTINGS_NESTED_KEY_VALUE_PATTERN, SETTINGS_ROOT_KEY_VALUE_PATTERN,
 };