forward plugin install failures to manager from sub-agents

install_configured now takes an optional notify recipient. on a
non-zero or spawn-failed 'claude plugin install', sub-agents send
the spec + stderr to manager via the hyperhive socket; manager
passes None so it doesn't message itself. boot still proceeds either
way — notification is best-effort.

hive-ag3nt/src/bin/hive-m1nd.rs

@@ -61,7 +61,7 @@ async fn main() -> Result<()> {
             let login_state = Arc::new(Mutex::new(initial));
             let bus = Bus::new();
             let files = turn::TurnFiles::prepare(&cli.socket, &label, mcp::Flavor::Manager).await?;
-            plugins::install_configured().await;
+            plugins::install_configured(&cli.socket, None).await;
             tokio::spawn(web_ui::serve(
                 label,
                 port,