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/chat_composer.rs

@@ -108,6 +108,7 @@ pub(crate) struct ChatComposer {
     custom_prompts: Vec<CustomPrompt>,
     footer_mode: FooterMode,
     footer_hint_override: Option<Vec<(String, String)>>,
+    context_window_percent: Option<u8>,
 }
 
 /// Popup state – at most one can be visible at any time.
@@ -150,6 +151,7 @@ impl ChatComposer {
             custom_prompts: Vec::new(),
             footer_mode: FooterMode::ShortcutPrompt,
             footer_hint_override: None,
+            context_window_percent: None,
         };
         // Apply configuration via the setter to keep side-effects centralized.
         this.set_disable_paste_burst(disable_paste_burst);
@@ -1317,6 +1319,7 @@ impl ChatComposer {
             esc_backtrack_hint: self.esc_backtrack_hint,
             use_shift_enter_hint: self.use_shift_enter_hint,
             is_task_running: self.is_task_running,
+            context_window_percent: self.context_window_percent,
         }
     }
 
@@ -1447,6 +1450,12 @@ impl ChatComposer {
         self.is_task_running = running;
     }
 
+    pub(crate) fn set_context_window_percent(&mut self, percent: Option<u8>) {
+        if self.context_window_percent != percent {
+            self.context_window_percent = percent;
+        }
+    }
+
     pub(crate) fn set_esc_backtrack_hint(&mut self, show: bool) {
         self.esc_backtrack_hint = show;
         if show {