Expose /usage data to statusLine for Claude Max users

As a Claude Max subscriber, I'd like to see my usage limits and remaining quota in the status line.

Current behavior

• The /usage command shows plan usage limits and rate limit status
• The statusLine feature only has access to: context_window, model, cost, workspace, version, output_style
• cost data is not relevant for Max subscribers (no per-token billing)
• Max users have no way to see their usage limits in the status line

Requested behavior

Add a usage or plan_usage field to the statusLine JSON input that includes:

• Current usage (requests/tokens used in current period)
• Usage limits (daily/monthly caps)
• Rate limit status
• Reset time

Example JSON structure

{
  "plan_usage": {
    "used": 150,
    "limit": 500,
    "percentage": 30,
    "resets_at": "2024-01-24T00:00:00Z",
    "rate_limited": false
  }
}

Use case

This would allow Max users to monitor their usage at a glance without running /usage repeatedly, helping them pace their usage throughout the day.