fix(sandboxes): sync runtime billing API contract

docs/sandboxes.md

@@ -153,3 +153,17 @@ its sessions and requires the `sandboxes.terminate` permission. History page
 limits range from 1 to 100. Archives are limited to 32 MiB compressed and expanded.
 
 Custom deployments accept at most 16 unique ports, from 1 through 65532.
+
+## Runtime billing
+
+Confirmed cloud running time is measured using baseline memory times observed
+running duration and consumes the account's Sandbox allowance. Paid overage is
+purchased in 5 GBh blocks at 25 credits per block; unused purchased capacity
+carries forward. Usage is not rounded up to a whole hour per session.
+Suspended and uncertain time are uncharged, snapshot costs are included, and
+burst resources are not billed separately. Local sessions consume no cloud
+allowance or credits.
+
+Plan and allowance refusals use `sandbox_plan_limit_exceeded` and
+`sandbox_allowance_exhausted`. Retrying these errors with a fresh request ID
+does not bypass the plan or account limit.