[Feature Request] Add context window token metrics to status line JSON

It would be useful to include remaining context window information in the JSON data passed to the status line command. Currently, the status line receives model info, cost tracking, and session metadata, but there's no way to display context usage.

Suggested additions to the status line JSON:

• context.used - tokens consumed in current context
• context.remaining - tokens remaining before context limit
• context.total - total context window size for current model
• context.percentage - percentage of context used

This would allow users to monitor context consumption directly in their terminal without running /cost, which is helpful for long sessions or when working with large codebases.