Show context window usage while tasks run (#4536)

## Summary
- show the remaining context window percentage in `/status` alongside
existing token usage details
- replace the composer shortcut prompt with the context window
percentage (or an unavailable message) while a task is running
- update TUI snapshots to reflect the new context window line

## Testing
- cargo test -p codex-tui

------
https://chatgpt.com/codex/tasks/task_i_68dc6e7397ac8321909d7daff25a396c

codex-rs/tui/src/bottom_pane/mod.rs

@@ -68,6 +68,7 @@ pub(crate) struct BottomPane {
     status: Option<StatusIndicatorWidget>,
     /// Queued user messages to show under the status indicator.
     queued_user_messages: Vec<String>,
+    context_window_percent: Option<u8>,
 }
 
 pub(crate) struct BottomPaneParams {
@@ -100,6 +101,7 @@ impl BottomPane {
             status: None,
             queued_user_messages: Vec::new(),
             esc_backtrack_hint: false,
+            context_window_percent: None,
         }
     }
 
@@ -341,6 +343,16 @@ impl BottomPane {
         }
     }
 
+    pub(crate) fn set_context_window_percent(&mut self, percent: Option<u8>) {
+        if self.context_window_percent == percent {
+            return;
+        }
+
+        self.context_window_percent = percent;
+        self.composer.set_context_window_percent(percent);
+        self.request_redraw();
+    }
+
     /// Show a generic list selection view with the provided items.
     pub(crate) fn show_selection_view(&mut self, params: list_selection_view::SelectionViewParams) {
         let view = list_selection_view::ListSelectionView::new(params, self.app_event_tx.clone());