Move releases to GitHub, with one bridge release on Gitea

GitHub has no generic package registry, so releases become GitHub
release assets. The updater and install.sh read releases/latest/download
and releases/download/<v>/, plain redirects with no API rate limit.
make release publishes a draft with all assets, then makes it latest.
make release-gitea publishes the same version to the old Gitea registry
for binaries that still update from there, and refuses until GitHub
serves it. See D33.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -38,7 +38,7 @@ the questions you actually have:
 ## Install
 
 ```sh
-curl -fsSL https://git.simon-franken.de/simonfranken/submarine/raw/branch/main/install.sh | sh
+curl -fsSL https://raw.githubusercontent.com/simonfranken/submarine/main/install.sh | sh
 ```
 
 Installs `sub` and `submarine` (linux/macOS, amd64/arm64) to `~/.local/bin`.