refactor(ci): organize workflow scripts and gate publish on main

scripts/github/common/check-tagged-commit-on-main.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+TARGET_SHA="${TARGET_SHA:-${GITHUB_SHA:?GITHUB_SHA must be set}}"
+
+git fetch --no-tags origin main
+
+if git merge-base --is-ancestor "${TARGET_SHA}" "origin/main"; then
+  echo "is_on_main=true" >> "$GITHUB_OUTPUT"
+  echo "Target commit ${TARGET_SHA} is contained in origin/main."
+else
+  echo "is_on_main=false" >> "$GITHUB_OUTPUT"
+  echo "Target commit ${TARGET_SHA} is not contained in origin/main. Skipping main-only action."
+fi