hive-ag3nt: echo turn (placeholder until claude integration)

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

@@ -62,6 +62,18 @@ async fn serve(socket: &std::path::Path, interval: Duration) -> Result<()> {
         match client::request(socket, AgentRequest::Recv).await {
             Ok(AgentResponse::Message { from, body }) => {
                 tracing::info!(%from, %body, "inbox");
+                // Placeholder "turn": echo back, prefixed. Phase 3c replaces this
+                // with `claude --print` once API-key plumbing exists. Don't echo
+                // an echo, so a manual `send` produces exactly one reply.
+                if !body.starts_with("echo: ") {
+                    let reply = AgentRequest::Send {
+                        to: from.clone(),
+                        body: format!("echo: {body}"),
+                    };
+                    if let Err(e) = client::request(socket, reply).await {
+                        tracing::warn!(error = ?e, "send reply failed");
+                    }
+                }
             }
             Ok(AgentResponse::Empty) => {}
             Ok(AgentResponse::Ok) => {