Remove constantly-updating context usage indicator from UI

What's the problem?

The UI shows a live context usage indicator like:

context: 11.9% (31.2k/262.1k)

This value updates continuously as the session progresses, and the constant flicker is visually distracting / annoying during normal use. It also feels redundant in the UI — most of the time I don't need to watch it tick up token by token.

Suggested fix

Remove the always-on context usage indicator from the default UI.

If the information is still considered useful, surface it only on demand instead of continuously, e.g.:

• a /context (or /usage) slash command that prints the current usage, or
• show it only when usage crosses a threshold (e.g. >80%) where the user actually needs to act.

Why it matters

• Reduces visual noise in the terminal UI.
• Avoids drawing attention to a metric the user doesn't need at every keystroke.
• Aligns with the "quiet by default, informative on demand" pattern most CLI tools follow.

Start by locating the CLI UI code that renders the continuously updating context usage indicator and trace how its value is refreshed. Confirm whether the intended behavior is removal from the default UI or an on-demand/threshold display, then update the relevant UI tests so they verify the indicator is no longer always shown.