pr merge fetches the head into a temporary branch, merges, and deletes the branch. After a squash or rebase merge the commits behind merge-head are on no ref: git gc prunes them in the workspace, and in the bucket nothing expands the short. GITREVIEW.md 1.5 names merge-base..merge-head as the only durable record of the diff. The merge writes refs/gitmsg/review/merged/<prShort> at the full merge head before it deletes the temporary branch. The state-ref push, its tracking mirror, the fetch of refs/gitmsg/* and the thin-push rule carry it. The site app expands merge-head from the manifest entry. Spec sentence first (SHOULD), then the rows in ARCHITECTURE.md and REVIEW.md.