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/footer.rs

@@ -5,6 +5,7 @@ use ratatui::buffer::Buffer;
 use ratatui::layout::Rect;
 use ratatui::style::Stylize;
 use ratatui::text::Line;
+use ratatui::text::Span;
 use ratatui::widgets::WidgetRef;
 use std::iter;
 
@@ -14,6 +15,7 @@ pub(crate) struct FooterProps {
     pub(crate) esc_backtrack_hint: bool,
     pub(crate) use_shift_enter_hint: bool,
     pub(crate) is_task_running: bool,
+    pub(crate) context_window_percent: Option<u8>,
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -75,7 +77,13 @@ fn footer_lines(props: FooterProps) -> Vec<Line<'static>> {
         FooterMode::CtrlCReminder => vec![ctrl_c_reminder_line(CtrlCReminderState {
             is_task_running: props.is_task_running,
         })],
-        FooterMode::ShortcutPrompt => vec![dim_line(indent_text("? for shortcuts"))],
+        FooterMode::ShortcutPrompt => {
+            if props.is_task_running {
+                vec![context_window_line(props.context_window_percent)]
+            } else {
+                vec![dim_line(indent_text("? for shortcuts"))]
+            }
+        }
         FooterMode::ShortcutOverlay => shortcut_overlay_lines(ShortcutsState {
             use_shift_enter_hint: props.use_shift_enter_hint,
             esc_backtrack_hint: props.esc_backtrack_hint,
@@ -211,6 +219,21 @@ fn dim_line(text: String) -> Line<'static> {
     Line::from(text).dim()
 }
 
+fn context_window_line(percent: Option<u8>) -> Line<'static> {
+    let mut spans: Vec<Span<'static>> = Vec::new();
+    spans.push(indent_text("").into());
+    match percent {
+        Some(percent) => {
+            spans.push(format!("{percent}%").bold());
+            spans.push(" context left".dim());
+        }
+        None => {
+            spans.push("? for shortcuts".dim());
+        }
+    }
+    Line::from(spans)
+}
+
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 enum ShortcutId {
     Commands,
@@ -398,6 +421,7 @@ mod tests {
                 esc_backtrack_hint: false,
                 use_shift_enter_hint: false,
                 is_task_running: false,
+                context_window_percent: None,
             },
         );
 
@@ -408,6 +432,7 @@ mod tests {
                 esc_backtrack_hint: true,
                 use_shift_enter_hint: true,
                 is_task_running: false,
+                context_window_percent: None,
             },
         );
 
@@ -418,6 +443,7 @@ mod tests {
                 esc_backtrack_hint: false,
                 use_shift_enter_hint: false,
                 is_task_running: false,
+                context_window_percent: None,
             },
         );
 
@@ -428,6 +454,7 @@ mod tests {
                 esc_backtrack_hint: false,
                 use_shift_enter_hint: false,
                 is_task_running: true,
+                context_window_percent: None,
             },
         );
 
@@ -438,6 +465,7 @@ mod tests {
                 esc_backtrack_hint: false,
                 use_shift_enter_hint: false,
                 is_task_running: false,
+                context_window_percent: None,
             },
         );
 
@@ -448,6 +476,18 @@ mod tests {
                 esc_backtrack_hint: true,
                 use_shift_enter_hint: false,
                 is_task_running: false,
+                context_window_percent: None,
+            },
+        );
+
+        snapshot_footer(
+            "footer_shortcuts_context_running",
+            FooterProps {
+                mode: FooterMode::ShortcutPrompt,
+                esc_backtrack_hint: false,
+                use_shift_enter_hint: false,
+                is_task_running: true,
+                context_window_percent: Some(72),
             },
         );
     }