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/zed_actions/src/lib.rs

@@ -190,12 +190,16 @@ pub mod agent {
 }
 
 pub mod assistant {
-    use gpui::{actions, impl_action_with_deprecated_aliases, impl_actions};
+    use gpui::{
+        action_with_deprecated_aliases, actions, impl_action_with_deprecated_aliases, impl_actions,
+    };
     use schemars::JsonSchema;
     use serde::Deserialize;
     use uuid::Uuid;
 
-    actions!(assistant, [ToggleFocus, ShowConfiguration]);
+    action_with_deprecated_aliases!(agent, ToggleFocus, ["assistant::ToggleFocus"]);
+
+    actions!(assistant, [ShowConfiguration]);
 
     #[derive(PartialEq, Clone, Default, Debug, Deserialize, JsonSchema)]
     #[serde(deny_unknown_fields)]
@@ -205,9 +209,12 @@ pub mod assistant {
     }
 
     impl_action_with_deprecated_aliases!(
-        assistant,
+        agent,
         OpenRulesLibrary,
-        ["assistant::DeployPromptLibrary"]
+        [
+            "assistant::OpenRulesLibrary",
+            "assistant::DeployPromptLibrary"
+        ]
     );
 
     #[derive(Clone, Default, Deserialize, PartialEq, JsonSchema)]