Upload built source packages over SFTP with host-key verification
(Launchpad fingerprints pinned in host_keys.yml, ask-to-accept
otherwise), Launchpad account discovery (git config lp.user), and
pre-flight checks the upload queue itself never does: changes file
discovery/validation, PPA existence via the Launchpad API, target
series validity, and debian/control Section validity (sections
bundled in distro_info.yml). Upload log prevents duplicate uploads
unless --force.
