Expose rate limit / quota data in statusline JSON payload

Feature Request

The /usage command shows rate limit quota data:

• Current session — X% used
• Current week (all models) — X% used
• Current week (Sonnet only) — X% used

This data is not available in the statusline JSON payload, which currently exposes cost, context_window, model, and workspace fields.

Request

Add rate limit / quota percentages to the statusline JSON, e.g.:

{
  "rate_limits": {
    "session": { "used_percentage": 9, "resets_at": "2026-03-19T14:30:00" },
    "weekly": { "used_percentage": 11, "resets_at": "2026-03-24T22:30:00" },
    "weekly_sonnet": { "used_percentage": 2, "resets_at": "2026-03-25T04:30:00" }
  }
}

Why

The statusline is great for at-a-glance info, but the one thing I keep opening /usage for is quota status. Having it in the statusline payload would let users display it alongside cost and context usage without needing a separate dialog.

The data already exists internally (likely from x-ratelimit-* API response headers) — it just needs to be forwarded to the statusline JSON.