Expose /usage data in statusLine JSON API

Feature Request

The statusLine command type receives a JSON payload on stdin with session-level metrics (cost.total_cost_usd, context_window.*, etc.), which is great for session tracking.

However, the /usage command shows additional account-level information that would be very useful in the status line but isn't currently available in the JSON payload:

Missing fields

• Weekly/monthly usage totals - cumulative cost and token usage across sessions for the current billing period
• Plan limits - remaining quota or usage cap information
• Cache token breakdown - cache_creation_input_tokens and cache_read_input_tokens are available under context_window.current_usage but only for the last API call, not as cumulative session totals

Use case

Users who want to monitor their usage budget in real-time via the status line currently have no way to see weekly/monthly totals without running /usage manually. A workaround of logging session costs to a file is inaccurate since it can't account for sessions not tracked (e.g. different machines, API usage outside CLI).

Proposed additions to the statusLine JSON

{
  "usage": {
    "weekly_cost_usd": 12.34,
    "monthly_cost_usd": 45.67,
    "plan_limit_usd": 100.00,
    "remaining_usd": 54.33
  },
  "cost": {
    "total_cache_creation_tokens": 50000,
    "total_cache_read_tokens": 120000
  }
}

This would allow status line scripts to show a complete usage picture matching what /usage already exposes, without needing workarounds.