[Feature Request] Display per-interaction cost delta in /cost command

Feature Request: Show cost delta per interaction

Currently, the /cost command shows cumulative session costs, which requires manual tracking to determine individual interaction costs.

Requested feature: Display the cost of each individual prompt/interaction, not just the running total.

Use case: I want to understand how much each specific task costs me without having to manually calculate deltas (current total minus previous total). This would help with:

• Understanding which types of tasks are more expensive
• Budgeting and cost awareness
• Learning which approaches are more cost-effective

Suggested implementations:

1. Auto-display cost delta at the end of each assistant response (e.g., "This interaction: $0.49")
2. Add a /cost --last or /cost --delta flag to show only the most recent interaction cost
3. Show both cumulative and delta in /cost output:

• Last interaction: $0.49
• Session total: $2.37

Thank you for considering this feature!

Environment Info

• Platform: darwin
• Terminal: androidstudio
• Version: 2.1.42
• Feedback ID: 15f148a7-3c44-41b2-bdb7-95528c8f23ed