Avoids the ls-remote + merge-base --is-ancestor walk in last() for the
common case by reading the recorded tag from .opencode-version. merge.ts
writes the file as part of the pre-merge compat commit so future runs of
fix-kilocode-markers, reset-to-upstream, and find-reset-candidates can
resolve the base tag instantly. Seeded with v1.14.33 (PR #9978).
