Monthly Usage Tracking

### Describe the feature or problem you'd like to solve

Show total usage of premium requests in TUi

### Proposed solution

Add /usage --monthly or /usage --billing-period to show aggregate premium request usage across all sessions for the current billing cycle.
Currently /usage only shows per-session stats, but the data exists in session.shutdown events in events.jsonl.

A monthly view would help users track spending against their allowance without visiting GitHub.com.

### Example prompts or workflows

_No response_

### Additional context

_No response_