Add rate limit utilization to status line JSON

Feature request

Add plan usage / rate limit utilization data to the status line JSON input so custom status line scripts can display it.

Context

The status line command receives JSON on stdin with cost, context_window, model, and workspace data. However, there's no way to show plan usage limits (session % and weekly %) — the same data shown on https://claude.ai/settings/usage.

Claude Code already parses these values from API response headers internally:

• anthropic-ratelimit-unified-5h-utilization (session/5-hour usage %)
• anthropic-ratelimit-unified-7d-utilization (weekly usage %)

But they aren't passed through to the status line JSON.

Proposed addition

Add a rate_limit object to the status line JSON:

{
  "rate_limit": {
    "session": {
      "utilization": 30,
      "resets_at": 1773435600
    },
    "weekly": {
      "utilization": 75,
      "resets_at": 1773820800
    },
    "status": "allowed"
  }
}

Why

• The data already exists in memory (parsed from every API response header)
• Zero additional API calls needed — just pass through what's already there
• Enables users to build status lines that match the claude.ai usage page, with progress bars showing session and weekly limits
• Currently the only percentage available is context_window.used_percentage, which measures something completely different (conversation context, not plan usage)

Current workaround

None viable. The OAuth token can't be used against /v1/messages to get the headers directly, and /api/oauth/usage returns "OAuth authentication is currently not supported." The claude.ai usage page requires browser authentication, making it impractical to scrape.