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/settings.rs

@@ -71,3 +71,25 @@ pub const SETTINGS_ASSISTANT_TOOLS_PATTERN: &str = r#"(document
     (#eq? @profiles "profiles")
     (#eq? @tools_key "tools")
 )"#;
+
+pub const SETTINGS_ASSISTANT_PATTERN: &str = r#"(document
+    (object
+        (pair
+            key: (string (string_content) @key)
+        )
+    )
+    (#eq? @key "assistant")
+)"#;
+
+pub const SETTINGS_EDIT_PREDICTIONS_ASSISTANT_PATTERN: &str = r#"(document
+    (object
+        (pair
+            key: (string (string_content) @edit_predictions)
+            value: (object
+                (pair key: (string (string_content) @enabled_in_assistant))
+            )
+        )
+    )
+    (#eq? @edit_predictions "edit_predictions")
+    (#eq? @enabled_in_assistant "enabled_in_assistant")
+)"#;