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/chatwidget.rs

@@ -396,8 +396,16 @@ impl ChatWidget {
     }
 
     pub(crate) fn set_token_info(&mut self, info: Option<TokenUsageInfo>) {
-        if info.is_some() {
-            self.token_info = info;
+        if let Some(info) = info {
+            let context_window = info
+                .model_context_window
+                .or(self.config.model_context_window);
+            let percent = context_window.map(|window| {
+                info.last_token_usage
+                    .percent_of_context_window_remaining(window)
+            });
+            self.bottom_pane.set_context_window_percent(percent);
+            self.token_info = Some(info);
         }
     }
 
@@ -1555,16 +1563,16 @@ impl ChatWidget {
     }
 
     pub(crate) fn add_status_output(&mut self) {
-        let default_usage;
-        let usage_ref = if let Some(ti) = &self.token_info {
-            &ti.total_token_usage
+        let default_usage = TokenUsage::default();
+        let (total_usage, context_usage) = if let Some(ti) = &self.token_info {
+            (&ti.total_token_usage, Some(&ti.last_token_usage))
         } else {
-            default_usage = TokenUsage::default();
-            &default_usage
+            (&default_usage, Some(&default_usage))
         };
         self.add_to_history(crate::status::new_status_output(
             &self.config,
-            usage_ref,
+            total_usage,
+            context_usage,
             &self.conversation_id,
             self.rate_limit_snapshot.as_ref(),
         ));