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/agent/src/agent_diff.rs

@@ -219,7 +219,7 @@ impl AgentDiffPane {
             .thread
             .read(cx)
             .summary()
-            .unwrap_or("Assistant Changes".into());
+            .unwrap_or("Agent Changes".into());
         if new_title != self.title {
             self.title = new_title;
             cx.emit(EditorEvent::TitleChanged);
@@ -473,7 +473,7 @@ impl Item for AgentDiffPane {
             .thread
             .read(cx)
             .summary()
-            .unwrap_or("Assistant Changes".into());
+            .unwrap_or("Agent Changes".into());
         Label::new(format!("Review: {}", summary))
             .color(if params.selected {
                 Color::Default