Use agent name as prompt in --agent --chat mode (e.g., "butler>")

Changed run_interactive() parameter from bool to Option<&str> agent_name.
When agent_name is Some, use it as the prompt instead of "g3>".

crates/g3-cli/src/accumulative.rs

@@ -300,7 +300,7 @@ async fn handle_command(
                 chat_combined_content,
                 workspace_dir,
                 cli.new_session,
-                false, // from_agent_mode
+                None, // agent_name (not in agent mode)
             )
             .await?;