release: Refactor release tracks for Linux packages and Docker

Support 4 release tracks instead of 2:
- sing-box / latest (stable release)
- sing-box-beta / latest-beta (stable pre-release)
- sing-box-testing / latest-testing (testing branch)
- sing-box-oldstable / latest-oldstable (oldstable branch)

Track is detected via git branch --contains and git tag,
replacing the old version-string hyphen check.

.github/detect_track.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+branches=$(git branch -r --contains HEAD)
+if echo "$branches" | grep -q 'origin/stable'; then
+  track=stable
+elif echo "$branches" | grep -q 'origin/testing'; then
+  track=testing
+elif echo "$branches" | grep -q 'origin/oldstable'; then
+  track=oldstable
+else
+  echo "ERROR: HEAD is not on any known release branch (stable/testing/oldstable)" >&2
+  exit 1
+fi
+
+if [[ "$track" == "stable" ]]; then
+  tag=$(git describe --tags --exact-match HEAD 2>/dev/null || true)
+  if [[ -n "$tag" && "$tag" == *"-"* ]]; then
+    track=beta
+  fi
+fi
+
+case "$track" in
+  stable)    name=sing-box;           docker_tag=latest ;;
+  beta)      name=sing-box-beta;      docker_tag=latest-beta ;;
+  testing)   name=sing-box-testing;   docker_tag=latest-testing ;;
+  oldstable) name=sing-box-oldstable; docker_tag=latest-oldstable ;;
+esac
+
+echo "track=${track} name=${name} docker_tag=${docker_tag}" >&2
+echo "TRACK=${track}" >> "$GITHUB_ENV"
+echo "NAME=${name}" >> "$GITHUB_ENV"
+echo "DOCKER_TAG=${docker_tag}" >> "$GITHUB_ENV"