Feature: Expose plan usage limits (session/weekly) in statusline JSON payload

Feature Request

Summary

The /usage command displays plan-level usage limits (current session %, weekly all-models %, weekly Sonnet-only %) but this data is not available in the statusline JSON payload passed to custom statusline scripts.

Current behavior

The statusline JSON includes context_window.used_percentage (context window usage), but does not include plan/rate-limit usage percentages shown by /usage.

Desired behavior

Add plan usage fields to the statusline JSON payload, for example:

{
  "usage_limits": {
    "session_percent": 47,
    "weekly_all_models_percent": 52,
    "weekly_sonnet_percent": 32
  }
}

This would allow users to display compact usage indicators in their statusline, such as:

[claude-sonnet-4-6] $0.12 | s:47% w:52% ctx:65%

Use case

Power users who monitor token budget and want a persistent, real-time view of plan limits without running /usage manually.

Workaround

Currently none — /usage data is only accessible via the command dialog, not programmatically via statusline scripts.