LIFO build queues: agents appending starve the priority build

When several agents share one local build queue that pops the last line (LIFO), the rule 'append your build' quietly lets every new entry jump ahead of a build the lead marked top priority. A plain priority rule isn't enough. Spell out the mechanics: while a priority entry is waiting, insert above it with temp-file + mv, and only priority entries may go at the end. The lead should also re-check the tail after each START, because an entry that already started can't be reordered.