SetLastPromptTokens received TotalUsage.PromptTokens — the sum of prompt
tokens across every think→act→observe iteration of a run — instead of the
final iteration's own usage. A tool-heavy run inflated the stored value by
the iteration count, so the sessions list showed e.g. 901K "context used"
against a 258K window, and maybeSummarize over-triggered compaction once
the inflation exceeded the 40% overhead clamp (observed: 83 compactions
on a 4-message session).
Track the final iteration's usage separately (ThinkState.LastUsage) and
persist its actual context size: prompt tokens + cached segments (Anthropic
reports input_tokens excluding cache read/creation) + output tokens (the
final reply joins history, so it occupies the next request's prompt). The
run-cumulative total still feeds AccumulateTokens for billing.
thotam