Add a Gitea release workflow

Releases only existed on GitHub, while install.sh now defaults to pulling the
archive from the Gitea mirror. This builds the same archive on a v* tag and
publishes it through the Gitea API, since the runner has no gh CLI: create the
release, fall back to looking it up by tag when it already exists, then upload
the tarball as an asset.

Two details are easy to get wrong and are commented in place. The release id is
the first "id" in Gitea's JSON, so the match must be non-greedy — a greedy one
picks up the author or asset id instead. And the id helper needs `|| true`,
because an empty grep under pipefail would abort the step on the path where the
release already exists.

The workflow cannot run yet: the Gitea repo is a pull mirror, which neither
runs Actions nor accepts release creation. It starts working once the repo is
un-mirrored, Actions are enabled and a runner with the ubuntu-latest label is
registered; the header of the file lists those prerequisites.

README.md

@@ -58,6 +58,11 @@ AWG_GEN=auto bash <(curl -Ls https://git.ma7neko.ru/maeneko/forgetting/raw/branc
 так что подходит любой из них. Если архив не найден в основном источнике,
 установщик предупреждает и берёт его из запасного.
 
+Архив собирается по тегу `v*`: на GitHub — `.github/workflows/release.yml`, в
+Gitea — `.gitea/workflows/release.yml` (сборка та же, публикация через API
+Gitea). Для запуска в Gitea репозиторий не должен быть pull-зеркалом, Actions
+включены, а runner зарегистрирован с меткой `ubuntu-latest`.
+
 ```bash
 REPO_BASE=https://git.example.org/me/vpn BRAND="My VPN" \
   bash <(curl -Ls https://git.example.org/me/vpn/raw/branch/main/install.sh)