On session resume, turns/inputTokens/outputTokens were cumulative
(entire session history) instead of per-step increments. Now we
snapshot metrics before prompt, compare after, and report the delta.
Changes:
- acp-client: add getSessionId() accessor
- hermes: extract snapshotUsage() + computeUsageDelta() pure functions
- hermes: runPrompt/runHermes/continueHermes use before/after snapshots
- 9 new unit tests for usage delta computation
Refs #68
