Show context usage in plan mode approval prompt

Summary

When exiting plan mode, the approval prompt offers "Clear context and auto-accept edits" vs "Auto-accept edits" — but there's no visibility into current context usage to inform that decision.

The status line (which can show context %) disappears when this modal prompt appears, and there's no way to run /context at that point either.

Request

Show the context window usage (e.g. 42k/200k tokens (21%)) somewhere in the plan mode approval prompt, so users can make an informed choice about whether clearing context is worthwhile.

Use Case

When context usage is low (say 25%), clearing context throws away useful conversation history for no benefit. When it's high (80%+), clearing makes sense. Right now you have to guess or remember to run /context before the prompt appears.