cost-counterfactual

Multi-baseline counterfactual cost analysis. Pairs with the existing observability surface:

• cost-budget-check — "have we crossed a threshold?" (reactive)
• cost-projection — "when will we cross a threshold?" (predictive)
• cost-counterfactual — "is the routing earning its keep?" (comparative) ← this one

Algorithm

1. Read all session-* records from the cost-tracking namespace.
2. Apply --since window filter (default all-time).
3. Sum tokens across byModel[*] entries for each session.
4. For each requested baseline (default: all three):
   • counterfactualUsd = (input × tier.input + output × tier.output + cache_write × tier.cache_write + cache_read × tier.cache_read) / 1M
5. Compute savings = counterfactualUsd − actualUsd.
6. Emit per-baseline totals + savings % across the comparison set.

Smoke transcript (2 sessions: 50K haiku tokens + 50K sonnet tokens)