Expose plan usage limits in statusline JSON input

Feature Request

Add account-level plan usage data to the JSON input that gets piped to the statusline command script.

What I'd like

Two new fields in the statusline JSON:

{
  "plan_usage": {
    "used_percentage": 14,
    "resets_in_seconds": 14340
  }
}

This matches the data already shown on claude.ai → Settings → Usage ("Current session: X% used, Resets in Y hr Z min").

Why

The statusline already exposes session-level context window usage (context_window.used_percentage), but there's no way to see account-level rate limit status without leaving the CLI to check the web UI. Having both metrics side-by-side in the statusline would help users pace their usage and avoid unexpected rate limits.

Current workaround

None — the data isn't available via any API endpoint, CLI command, or programmatic access method.