Add current-week and current-month usage commands

[vycdev]

Goal

Add focused usage commands for the current calendar week and current calendar month.

Commands

• !usage week — show usage from Monday at 00:00 through now.
• !usage month — show usage from the first day of the current month at 00:00 through now.

Keep the existing !usage monthly command unchanged; it should continue showing historical monthly totals and trends.

Expected output

Each command should return a concise aggregate for its current period, including:

• total cost
• total tokens and token breakdown
• per-model breakdown
• the covered date range in the embed
• a clear empty state when no usage exists

Acceptance criteria

• !usage week includes only the current calendar week.
• !usage month includes only the current calendar month.
• Period boundaries use one consistent, documented timezone.
• The usage help embed lists both new commands and distinguishes month from monthly.
• Existing today, daily, blocks, and monthly behavior remains unchanged.
• Relevant tests are added and npm run build passes.