fix(chat): use correct environment variable for default shell detection

Author: GoodluckH

crates/q_cli/src/cli/chat/tools/execute_shell_commands.rs

@@ -92,7 +92,7 @@ impl ExecuteShellCommands {
 
     pub async fn invoke(&self, mut updates: impl Write) -> Result<InvokeOutput> {
         // Detect the user's default shell
-        let shell = std::env::var("$0").unwrap_or_else(|_| "bash".to_string());
+        let shell = std::env::var("SHELL").unwrap_or_else(|_| "bash".to_string());
 
         let home = std::env::var("HOME").unwrap_or_else(|_| "~".to_string());
         let shell_init_file = match shell.as_str() {