Fix /goal usage text for control commands (#26551)

## Why

The TUI's `/goal` usage text only advertised the objective form even
though `/goal clear`, `/goal edit`, `/goal pause`, and `/goal resume`
are implemented. This made the lifecycle controls difficult to discover
and allowed the duplicated help text to drift from actual behavior.

Fixes #25530.

## What changed

- Show the complete `/goal [<objective>|clear|edit|pause|resume]` syntax
in usage messages.
- Share one usage string across slash-command dispatch and goal-related
app messages.
- Add inline snapshot coverage for the control-command usage path.

codex-rs/tui/src/chatwidget/tests/slash_commands.rs

@@ -784,6 +784,21 @@ async fn goal_control_slash_commands_emit_goal_events() {
     }
 }
 
+#[tokio::test]
+async fn goal_control_slash_command_without_thread_shows_full_usage() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.set_feature_enabled(Feature::Goals, /*enabled*/ true);
+
+    submit_composer_text(&mut chat, "/goal pause");
+
+    let cells = drain_insert_history(&mut rx);
+    assert_eq!(cells.len(), 1, "expected goal usage message");
+    insta::assert_snapshot!(
+        lines_to_single_string(&cells[0]),
+        @"• Usage: /goal [<objective>|clear|edit|pause|resume] The session must start before you can change a goal."
+    );
+}
+
 #[tokio::test]
 async fn goal_edit_slash_command_opens_goal_editor() {
     for thread_id in [Some(ThreadId::new()), None] {