The publish job used a fixed body that still read 'DRAFT - do not publish' on the
live release and listed none of the actual changes. Now the body is built from the
source commits that went into the binary: the build records which invisible_firefox
commit it came from (source-commit.txt), and publish diffs that against the previous
release's recorded commit via the GitHub compare API (no deep clone, no cross-repo
token) to list the user-facing subjects. docs/chore/ci/test commits are filtered out,
and the body ends with 'Built from invisible_firefox @<sha>' for traceability. It's
still a draft - the realness gate and the un-draft flip stay manual (issue #14).