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/snapshots/codex_tui__chatwidget__tests__status_widget_active.snap

@@ -1,6 +1,5 @@
 ---
 source: tui/src/chatwidget/tests.rs
-assertion_line: 1445
 expression: terminal.backend()
 ---
 "                                                                                "