cost.total_cost_usd does not reset when using /new

Description

When using /new to start a new session within the same Claude Code process, cost.total_cost_usd in the session JSON does not reset to 0. It continues accumulating from the previous session.

Expected behavior

cost.total_cost_usd should reset to 0 when /new is used, since it starts a new logical session (new transcript file, new conversation context).

Actual behavior

cost.total_cost_usd continues from the previous session's total, reflecting cumulative process cost rather than per-session cost.

Steps to reproduce

1. Use Claude Code for a bit and note the cost
2. Run /new
3. Observe that cost.total_cost_usd still reflects the cumulative process cost, not the new session cost