Activate selected agent on empty prefix

Once /agent  shows completion candidates, route Enter directly to the highlighted agent while preserving the bare /agent picker.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: b6dd7b04-d567-42e1-8866-257b10fa88a0

Author: vanzue

tools/wta/src/app.rs

@@ -47,16 +47,11 @@ fn agent_command_on_enter(
     input: &str,
     selected: Option<&AvailableAgent>,
 ) -> Option<ParsedCommand> {
-    let prefix = commands::agent_id_prefix(input)?;
-    let rest = if prefix.is_empty() {
-        String::new()
-    } else {
-        selected?.id.clone()
-    };
+    commands::agent_id_prefix(input)?;
     Some(ParsedCommand {
         kind: CommandKind::Agent,
         spec: commands::lookup("agent").expect("/agent is registered"),
-        rest,
+        rest: selected?.id.clone(),
     })
 }
 

tools/wta/src/slash_command_tests.rs

@@ -432,9 +432,9 @@ fn agent_trailing_space_opens_completion_with_all_agents() {
     let command =
         agent_command_on_enter(&app.current_tab().input, highlighted).expect("agent command");
     assert_eq!(command.kind, CommandKind::Agent);
-    assert!(
-        command.rest.is_empty(),
-        "Enter must dispatch bare /agent so cmd_agent opens the picker"
+    assert_eq!(
+        command.rest, "copilot",
+        "Enter must dispatch the highlighted agent once the completion list is visible"
     );
 }