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/assistant_settings/src/assistant_settings.rs

@@ -693,7 +693,7 @@ pub struct LegacyAssistantSettingsContent {
 }
 
 impl Settings for AssistantSettings {
-    const KEY: Option<&'static str> = Some("assistant");
+    const KEY: Option<&'static str> = Some("agent");
 
     const PRESERVED_KEYS: Option<&'static [&'static str]> = Some(&["version"]);
 
@@ -894,12 +894,12 @@ mod tests {
 
         #[derive(Debug, Deserialize)]
         struct AssistantSettingsTest {
-            assistant: AssistantSettingsContent,
+            agent: AssistantSettingsContent,
         }
 
         let assistant_settings: AssistantSettingsTest =
             serde_json_lenient::from_str(&raw_settings_value).unwrap();
 
-        assert!(!assistant_settings.assistant.is_version_outdated());
+        assert!(!assistant_settings.agent.is_version_outdated());
     }
 }