feat: expose rate limit data in statusline JSON

Problem

The statusline feature provides rich session data (context window, cost, model info) that can be used in tmux/terminal statuslines. However, rate limit information is not included, even though it's available internally from API response headers.

The /usage command shows rate limit data (remaining tokens, reset time) inside the TUI, but this data is only accessible within the running process — it's not persisted to disk or exposed to external scripts.

Current statusline JSON

{
  "cost": { "total_cost_usd": 17.82 },
  "context_window": { "used_percentage": 30, "remaining_percentage": 70 },
  "model": { "id": "claude-opus-4-6" }
}

Requested addition

{
  "rate_limit": {
    "tokens_remaining": 150000,
    "tokens_limit": 200000,
    "used_percentage": 25,
    "reset_at": "2026-02-16T18:00:00Z"
  }
}

The data is already available from Anthropic API response headers (anthropic-ratelimit-tokens-remaining, anthropic-ratelimit-tokens-reset) — it just needs to be forwarded to the statusline output.

Use case

I have a custom tmux statusline that shows context usage, per-model cost breakdown (Opus/Sonnet/Haiku), and accumulated spending (day/week/month). Adding rate limit visibility would complete the picture — letting me know when I'm approaching limits without having to run /usage interactively.

Why external access matters

• /usage only works inside an interactive session
• OAuth tokens don't work with the standard Messages API, so external scripts can't query rate limits independently
• Rate limit headers are not stored in debug logs, transcripts, or any cache file
• The statusline script is the natural place for this data since it already receives real-time session state

Environment

• Claude Code 2.1.42
• Max 5x plan
• Linux (tmux statusline via statusline_command setting)