Burn-rate trend observability. The fourth leg of the cost-tracker forward-cost stack:

Question	Skill
"Have we crossed a threshold?" (reactive)	`cost-budget-check`
"When will we cross a threshold?" (predictive)	`cost-projection`
"Could we have spent less?" (comparative)	`cost-counterfactual`
"Is daily burn ACCELERATING?" (trend)	`cost-burn` ← this

Algorithm

Read all session-* records from cost-tracking namespace.
Bin into --bucket duration windows (default 1d) over --lookback (default 14d).
For each bucket: {n: sessions, spendUsd: sum(total_cost_usd)}.
Compute delta = latest.spendUsd - mean(prior non-empty buckets).
If --alert-on-acceleration-pct N is set: exit 1 when deltaPct > N.

cost-burn