fix(ci): resolve GitHub username attribution in release changelog

Replace direct git log author formatting with GitHub API username
lookup for both stable and nightly release changelogs.

Each commit SHA is now resolved via `gh api` to retrieve the actual
GitHub username (author.login), producing proper @mentions in
release notes. Falls back to git author name without @ prefix when
the API lookup returns no match.

Changes applied to both the stable release and nightly release
changelog generation steps, with GH_TOKEN env var added for API
authentication.