[Feature Request] Add plan-level usage monitoring and threshold alerts

Bug Description
Feature request: Plan-level usage monitoring and threshold alerts

As a Claude Max subscriber using Claude Code as my primary interface, I have no visibility into how much of my plan's usage allocation I've consumed in the current billing
period. The only usage info available is session-level (/cost), which doesn't help me pace my usage across a billing cycle.

Requested features:

1. A command (e.g. /usage) that shows current plan consumption as a percentage of the billing period limit
2. A configurable threshold alert (e.g. at 80%) — either via the status line, a notification hook, or an in-session warning
3. Ideally, expose plan usage data to the status line so users can monitor it passively

This would help users avoid unexpectedly hitting rate limits mid-task and make it easier to distribute usage across a billing period.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.42
• Feedback ID: ab530b2e-cf7e-400f-866e-18f990484dd3

Errors

[{"error":"Error: NON-FATAL: Lock acquisition failed for /Users/mdoerfler/.local/share/claude/versions/2.1.42 (expected in multi-process scenarios)\n    at ZwR (/$bunfs/root/claude:3483:2098)\n    at ESA (/$bunfs/root/claude:3483:1209)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-16T21:27:41.502Z"}]