Feature Request: Show token usage persistently in statusline

Summary

Currently, token/usage information is only accessible via the /usage command, which opens a modal dialog that disappears after dismissal. There is no way to monitor usage at a glance during a session.

Proposed Feature

Add an option to display the current session's token usage percentage persistently in the statusline (the bottom bar), so users can monitor their usage at all times without interrupting their workflow.

Suggested Format

Something compact like:

[model: claude-sonnet-4-6] [branch: main] [usage: 5%]

Or a minimal indicator:

◆ 5% used

Motivation

• Power users who run long sessions want to know when they're approaching their limit before hitting it
• Constantly opening /usage interrupts the flow of work
• The statusline already exists and has space for this information

Alternatives Considered

• A configurable threshold warning (e.g., alert at 80%) — useful but not a replacement for always-visible info
• A separate persistent HUD panel — heavier than needed; statusline integration is more elegant