opencode v1.18.15: message-ordering fixes, compaction history retention, desktop locale coverage

Maintenance release centred on message chronology: ordering, revert/fork, and session timelines now key off real timestamps rather than message IDs, and repeated compaction keeps earlier tool-call history instead of dropping orphaned results. Desktop gains JSON transcript export and much wider locale coverage.

Tool: opencode · Version: v1.18.15 · Significance: 2/5 · fix, feature