[Feature Request] Show overall account usage / remaining credits in statusline

Summary

The statusline currently shows per-session context window usage and estimated session cost, but there's no way to see overall account-level usage or remaining budget without leaving the editor and going to the console.

What I'm asking for

A statusline variable (e.g. {{totalUsage}} or {{creditsRemaining}}) that surfaces monthly token consumption or remaining billing credits — similar to how other developer tools display a usage indicator inline.

Why it matters

• Checking usage mid-work requires breaking flow to visit platform.anthropic.com
• The per-session cost: $0.00 counter is useful but doesn't tell you where you stand against your monthly limit or quota
• Power users running many sessions back-to-back have no ambient awareness of spend

Suggested implementation

• Expose account usage via a lightweight API call (cached, not per-prompt) that Claude Code can poll
• Surface it as a statusline variable so users can opt in via ~/.claude/statusline.sh