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/status/card.rs

@@ -29,11 +29,19 @@ use super::rate_limits::compose_rate_limit_data;
 use super::rate_limits::format_status_limit_summary;
 use super::rate_limits::render_status_limit_progress_bar;
 
+#[derive(Debug, Clone)]
+struct StatusContextWindowData {
+    percent_remaining: u8,
+    tokens_in_context: u64,
+    window: u64,
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct StatusTokenUsageData {
     total: u64,
     input: u64,
     output: u64,
+    context_window: Option<StatusContextWindowData>,
 }
 
 #[derive(Debug)]
@@ -52,12 +60,13 @@ struct StatusHistoryCell {
 
 pub(crate) fn new_status_output(
     config: &Config,
-    usage: &TokenUsage,
+    total_usage: &TokenUsage,
+    context_usage: Option<&TokenUsage>,
     session_id: &Option<ConversationId>,
     rate_limits: Option<&RateLimitSnapshotDisplay>,
 ) -> CompositeHistoryCell {
     let command = PlainHistoryCell::new(vec!["/status".magenta().into()]);
-    let card = StatusHistoryCell::new(config, usage, session_id, rate_limits);
+    let card = StatusHistoryCell::new(config, total_usage, context_usage, session_id, rate_limits);
 
     CompositeHistoryCell::new(vec![Box::new(command), Box::new(card)])
 }
@@ -65,7 +74,8 @@ pub(crate) fn new_status_output(
 impl StatusHistoryCell {
     fn new(
         config: &Config,
-        usage: &TokenUsage,
+        total_usage: &TokenUsage,
+        context_usage: Option<&TokenUsage>,
         session_id: &Option<ConversationId>,
         rate_limits: Option<&RateLimitSnapshotDisplay>,
     ) -> Self {
@@ -84,10 +94,19 @@ impl StatusHistoryCell {
         let agents_summary = compose_agents_summary(config);
         let account = compose_account_display(config);
         let session_id = session_id.as_ref().map(std::string::ToString::to_string);
+        let context_window = config.model_context_window.and_then(|window| {
+            context_usage.map(|usage| StatusContextWindowData {
+                percent_remaining: usage.percent_of_context_window_remaining(window),
+                tokens_in_context: usage.tokens_in_context_window(),
+                window,
+            })
+        });
+
         let token_usage = StatusTokenUsageData {
-            total: usage.blended_total(),
-            input: usage.non_cached_input(),
-            output: usage.output_tokens,
+            total: total_usage.blended_total(),
+            input: total_usage.non_cached_input(),
+            output: total_usage.output_tokens,
+            context_window,
         };
         let rate_limits = compose_rate_limit_data(rate_limits);
 
@@ -123,6 +142,22 @@ impl StatusHistoryCell {
         ]
     }
 
+    fn context_window_spans(&self) -> Option<Vec<Span<'static>>> {
+        let context = self.token_usage.context_window.as_ref()?;
+        let percent = context.percent_remaining;
+        let used_fmt = format_tokens_compact(context.tokens_in_context);
+        let window_fmt = format_tokens_compact(context.window);
+
+        Some(vec![
+            Span::from(format!("{percent}% left")),
+            Span::from(" (").dim(),
+            Span::from(used_fmt).dim(),
+            Span::from(" / ").dim(),
+            Span::from(window_fmt).dim(),
+            Span::from(")").dim(),
+        ])
+    }
+
     fn rate_limit_lines(
         &self,
         available_inner_width: usize,
@@ -235,6 +270,9 @@ impl HistoryCell for StatusHistoryCell {
             push_label(&mut labels, &mut seen, "Session");
         }
         push_label(&mut labels, &mut seen, "Token usage");
+        if self.token_usage.context_window.is_some() {
+            push_label(&mut labels, &mut seen, "Context window");
+        }
         self.collect_rate_limit_labels(&mut seen, &mut labels);
 
         let formatter = FieldFormatter::from_labels(labels.iter().map(String::as_str));
@@ -266,6 +304,10 @@ impl HistoryCell for StatusHistoryCell {
         lines.push(Line::from(Vec::<Span<'static>>::new()));
         lines.push(formatter.line("Token usage", self.token_usage_spans()));
 
+        if let Some(spans) = self.context_window_spans() {
+            lines.push(formatter.line("Context window", spans));
+        }
+
         lines.extend(self.rate_limit_lines(available_inner_width, &formatter));
 
         let content_width = lines.iter().map(line_display_width).max().unwrap_or(0);