Add a persistent context window status bar

Feature request

Please add a clear, persistent context window status bar to Codex CLI so users can see current context usage at a glance during a session.

Claude Code exposes this kind of session/status information directly in the terminal UI, and it is extremely useful during long-running coding sessions. Codex should have the same level of visibility.

Why this matters

Right now, it is too hard to understand how close a Codex session is to filling the context window without interrupting workflow or checking secondary status output. For agentic coding sessions, context usage is operationally important:

• users need to know when a session is getting close to compaction
• users need to decide whether to continue, compact, or start a fresh session
• large repo work can silently consume context
• losing context mid-task can degrade output quality or cause confusing behavior

This should be visible continuously, not hidden behind a command.

Suggested behavior

Add a persistent footer/status bar item showing context window usage, for example:

• Context: 42% used
• Context: 58% left
• 249K / 998K tokens
• optionally a compact progress bar

Ideally this should be configurable through /statusline, but the default Codex CLI experience should expose context usage without requiring users to discover and configure it manually.

Expected result

Codex users should be able to glance at the terminal and immediately understand how much context remains, similar to Claude Code.

This feels like core UX for a coding agent with long-running sessions and large context windows.