Include <downloadkey prefix="dlid="> in Joomla update XML whenever
Update Server is enabled for the repo, not just when require_key is
set. This ensures Joomla shows the Download Key field in Update Sites
even when downloads are currently public.
Also corrected joomlaTagName comment with Joomla source reference.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>