Craft publish_repo 'self' sentinel resolves to GITHUB_REPOSITORY at runtime

The composite action's publish_repo input supports a special sentinel value "self" which resolves to $GITHUB_REPOSITORY at runtime in the bash script of the 'Request publish' step. This allows repos to create publish request issues in themselves rather than in a separate {owner}/publish repo. The resolution happens in bash (not in the GitHub Actions expression) because the expression layer sets PUBLISH_REPO via inputs.publish_repo || format('{0}/publish', github.repository_owner) — the string "self" passes through as-is and gets resolved to the actual repo name in the shell. Useful for personal/small repos where the default GITHUB_TOKEN already has write access to the repo itself.