Hardcoding origin/main broke in any checkout whose remote isn't named
origin (e.g. an agent worktree, whose remote is renamed to forge). Try
BASE_REF if set, else every configured remote in a stable order
(origin first to match CI, then the rest alphabetically), fetching each
remote's copy of the base branch if not already present. If nothing
resolves, exit 1 with a clear message rather than falling back to any
guessed range - an unresolvable base must never read as a clean pass.
Refs #4449
d726682be4