GITEA_URL as an env var does nothing in goreleaser v2; the giteaurls
config block is what routes scm releases to the Gitea API. Without it
the publish step fails with: scm releases: invalid URL: "".
Removes the release.github block — target and credentials come from
GITEA_URL/GITEA_TOKEN env. Pins the archive name_template to
agent-lib_<os>_<arch> so fleet download URLs stay predictable.