[FEATURE] Allow manual reset of session cost counter

Summary

It would be useful to be able to manually zero the session cost counter displayed in the status line, without having to end the session.

Use case

Long-running sessions that span multiple days accumulate cost from previous days in the counter, making it impossible to track today's spend without starting a fresh session and losing all conversation context.

Proposed solution

A command such as /reset-cost (or clicking/interacting with the cost display in the status line) that resets the counter to $0.00, so users can track spend across natural boundaries like days.

Alternatives considered

• Starting a new session — resets the counter but discards conversation context
• /clear — clears context but does not reset the cost display