Adds typed JSON build records under data/builds/<app>/<build-id>.{json,log} keyed on a stable base36 ULID-style DOKKU_BUILD_ID generated for every deploy. The new commands surface that history (builds:list, builds:info, builds:prune) and an operator-configurable retention via builds:set retention. The existing builds:cancel and builds:output now key on the build-id (with safe handling for already-finalized and abandoned records), and the per-build log file replaces journalctl as the durable source of truth for builds:output.
Custom nginx.conf.sigil templates that reference DOKKU_APP_WEB_LISTENERS
may now receive an empty value when rendered for apps without running web
processes. The migration guide documents how to handle this with a
conditional in the template.
