feat: add Linux/Unix scripts and Makefile entry

scripts/sync-items.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# SFCraft 文档 · 物品贴图自动同步 (Linux / macOS / Unix)
+# 用法: ./scripts/sync-items.sh [-f]
+set -euo pipefail
+
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+PY=""
+if command -v python3 >/dev/null 2>&1; then
+  PY="python3"
+elif command -v python >/dev/null 2>&1; then
+  PY="python"
+else
+  echo "[错误] 未找到 python3 / python, 请先安装 Python 3。" >&2
+  exit 1
+fi
+
+exec "$PY" "$DIR/sync-items.py" "$@"