ci: Feed RELEASE_NOTES.md into Sparkle appcast + GitHub release body

Single committed source of truth for release notes. CI renders it to
GFM HTML (gh /markdown) and embeds it in appcast.xml <description> so
the Sparkle update dialog shows notes, and sets the GitHub Release body
from the same file. xmllint gates a malformed feed; ]]> is sanitized;
softprops fail_on_unmatched_files:false. /release docs updated: write &
commit RELEASE_NOTES.md before tagging; bilingual EN/中文, no flag
headers; patch releases include the parent minor's notes.