Address review feedback from @Zhaoyikaiii:
- Replace map[string]*sync.Mutex + separate mu with sync.Map.LoadOrStore
for simpler, lock-free session lock management.
- Add skip parameter to readMessages so callers (GetHistory, Compact)
can skip truncated lines without paying the json.Unmarshal cost.
- Add countLines helper for TruncateHistory's count reconciliation,
avoiding full deserialization when only the line count is needed.
5d73ee2d9a