A relation is conflicted when two entries were recorded without seeing each other,
typically on two branches that were then merged. mix surfex.status lists the tips'
ids; this records the picked one again with both as parents (Surfex.Record.resolve/7).
mix surfex.resolve "spec.md#Carts/Adding items" MyApp.Cart.add/2 --type implements --pick 3f9c01--pick takes the start of a tip's id. If the code or spec has moved since, the
relation is then dangling, and mix surfex.confirm is next.