fix(tui): every slash submit echoes a user row

A command that clears the composer must leave what the user typed in
the thread: bare left-bar error lines with no user row read as a void
(commercial-bar finding 1/4). Message submits already echo in
dispatch_user_message_with_recovery; this covers the slash branch of
both the launch and session submit tails. Test to follow on the
headless-terminal seam (AppTerminal is Stdout-bound, so the existing
dispatchers are not directly callable from tests).

crates/tui/src/tui/ui/event_loop.rs

@@ -859,6 +859,10 @@ async fn dispatch_launch_composer_submit(
         return Ok(false);
     }
     if looks_like_slash_command_input(&input) {
+        // Every submit echoes (see submit_decided_composer_input).
+        app.add_message(HistoryCell::User {
+            content: input.clone(),
+        });
         if execute_command_input(
             terminal,
             app,
@@ -966,6 +970,12 @@ async fn submit_decided_composer_input(
         return Ok(false);
     }
     if looks_like_slash_command_input(&input) {
+        // Every submit echoes: a command that clears the composer must leave
+        // what the user typed in the thread, not just its receipt — bare
+        // error lines with no user row read as a void.
+        app.add_message(HistoryCell::User {
+            content: input.clone(),
+        });
         if execute_command_input(
             terminal,
             app,