Expose API usage percentages in statusline JSON

Feature Request

Currently the statusline JSON includes cost data (total_cost_usd, duration, etc) but doesn't expose API usage limit percentages that are visible in the UI.

Use case: Want to display session/weekly usage percentages in a custom statusline to track how close I am to rate limits.

Current workaround: None - the data isn't available in the JSON passed to statusline scripts.

Proposed solution: Add an apiUsage field to the statusline JSON with structure like:

{
  "apiUsage": {
    "session": {
      "percentUsed": 12.5,
      "tokensUsed": 25000,
      "tokensLimit": 200000
    },
    "weekly": {
      "percentUsed": 45.2,
      "tokensUsed": 904000,
      "tokensLimit": 2000000
    }
  }
}

This would enable users to create statuslines that show usage % with color coding (green/yellow/red) based on proximity to limits.