agent: Update message editor placeholder (#37441)

Release Notes:

- N/A

crates/agent_ui/src/acp/thread_view.rs

@@ -332,6 +332,11 @@ impl AcpThreadView {
 
         let placeholder = if agent.name() == "Zed Agent" {
             format!("Message the {} — @ to include context", agent.name())
+        } else if agent.name() == "Claude Code" || !available_commands.borrow().is_empty() {
+            format!(
+                "Message {} — @ to include context, / for commands",
+                agent.name()
+            )
         } else {
             format!("Message {} — @ to include context", agent.name())
         };