/usage label 'current session' is misleading for rate limit quota

Description

The /usage command displays rate limit / quota usage with the label "current session". This is misleading because it implies the quota resets when you restart Claude Code (i.e., start a new session). In reality, it's a server-side time window (typically ~5 hours) that runs independently of sessions.

Steps to reproduce

1. Use Claude Code until quota is high (e.g., 94%)
2. Run /usage — see "current session" label
3. Quit and restart Claude Code
4. Run /usage — quota is still at 94%

Expected behavior

The label should reflect that it's a time-based quota, not session-based. Something like "current rate limit window" or "quota (resets in X hours)" would be more accurate.

Why this matters

Users naturally assume "current session" = this Claude Code process. When they see high usage, their first instinct is to restart — which doesn't help and wastes time.