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/lib.rs

@@ -218,7 +218,7 @@ async fn run_console_mode(
             combined_content,
             project.workspace(),
             cli.new_session,
-            false, // from_agent_mode
+            None, // agent_name (not in agent mode)
         )
         .await
     }