In a path-filtered monorepo the branch tip is often another domain's
commit: a push tipping at an infra-only change made every backend deploy
(push-triggered or dispatched) compute sha-<tip> and go looking for an
image no build ever produced -- 5 minutes of pull retries, then a red run
(this bit backend-deploy-prod on the release tip today).
Build and deploy now compute the SAME key: git log -1 -- <domain>/, the
last commit that actually touched the domain. Builds tag with it, deploys
pull it, dispatches always resolve, and re-pushes of unrelated commits
are naturally idempotent (same key -> image already exists). Deploy
workflows gain a fetch-depth:0 checkout -- a depth-1 clone can't see past
an unrelated tip to find the key.
