assistant2: Rename assistant2 actions to agent (#27877)

This PR renames the `assistant2` actions to `agent`.

Note that any `assistant` actions have been left as-is for now so that
there aren't any changes to users not in the feature flag.

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -43,7 +43,7 @@ pub use crate::thread_store::ThreadStore;
 pub use assistant_diff::{AssistantDiff, AssistantDiffToolbar};
 
 actions!(
-    assistant2,
+    agent,
     [
         NewThread,
         NewPromptEditor,
@@ -87,9 +87,9 @@ impl ManageProfiles {
     }
 }
 
-impl_actions!(assistant, [ManageProfiles]);
+impl_actions!(agent, [ManageProfiles]);
 
-const NAMESPACE: &str = "assistant2";
+const NAMESPACE: &str = "agent";
 
 /// Initializes the `assistant2` crate.
 pub fn init(