fix: read token usage from ACP response instead of DB

Tokens (inputTokens, outputTokens) now come from ACP PromptResponse.usage
which is populated synchronously from run_conversation() — no WAL race.
Turns still come from DB before/after snapshot.

Previously both were read from hermes state.db after ACP prompt returned,
but WAL write lag caused incomplete token data (e.g. 235 vs actual 26,080).

Refs #91

packages/agent-hermes/src/index.ts

@@ -1,7 +1,8 @@
+export type { AcpUsage } from "./acp-client.js";
 export { HermesAcpClient } from "./acp-client.js";
 export {
   buildHermesPrompt,
-  computeUsageDelta,
+  buildUsage,
   createHermesAgent,
-  snapshotUsage,
+  snapshotTurns,
 } from "./hermes.js";