chore: drop Codeberg release workflow and document GitHub-only CI

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/ci-build-release.sh

@@ -4,10 +4,10 @@ set -euo pipefail
 # Build and package every release artifact on a Linux host that already has the
 # cross toolchain installed (native gcc + X11/OpenGL headers, the aarch64 cross
 # compiler, and the MinGW-w64 toolchain for the Windows GUI binary). This is the
-# non-Docker counterpart to scripts/build-release-linux.sh: the CI workflows in
-# .github/ and .forgejo/ install those packages directly on the runner and then
-# call this script, so the exact build/package commands live in one place and do
-# not drift between the two forges.
+# non-Docker counterpart to scripts/build-release-linux.sh: the GitHub Actions
+# release workflow installs those packages directly on the runner and then calls
+# this script, so the exact build/package commands live in one place and do not
+# drift from local release builds.
 #
 # The build flags mirror the other scripts intentionally: -trimpath strips local
 # paths, -s -w drops symbol/debug tables to shrink the binaries, -H=windowsgui