Thesis vocabulary lands: per-workspace default type, plus a domain.retype migration

Build-Order step 5, finished: humain-vc now defaults to 'thesis' on its own (no more typing it every session), and a new domain.retype capability moved the one pre-existing domain — consumer-immunology — from strategy to thesis, DB and filesystem both. Riding along: a real back button in Corpora Curator's header, and a directory-existence bug caught mid-migration and fixed before it shipped broken.