Expose account usage/rate limit percentage in statusline JSON input

Feature Request

Expose the account-level usage/rate limit percentage (the same data shown by /status → Usage → Current session) as a field in the JSON input that gets piped to custom statusline scripts.

Use Case

Users with custom statuslines can display context window percentage, session cost, tokens, etc. — but the most actionable metric for subscription users (Claude Max/Pro) is how much of their daily rate limit they've used. Currently this is only visible by manually running /status inside an active session.

Proposed Solution

Add a field to the statusline JSON input, something like:

{
  "account": {
    "usage_percentage": 41,
    "resets_at": "2026-02-18T00:00:00Z"
  }
}

This would allow statusline scripts to display the quota percentage alongside existing metrics like context window usage and session cost.

Current Workaround

None. The rate limit data is only accessible through the authenticated /status command inside a running session. External scripts (including the statusline script) cannot access it.