支持 WorkBuddy 套件市场

- .claude-plugin/marketplace.json 同时充当 WorkBuddy 市场清单，市场内含 engineering-conventions 套件，source 指向仓库根
- README 补充 WorkBuddy 安装路径与市场源地址，版本号说明由三处改为四处

.claude-plugin/marketplace.json

@@ -0,0 +1,15 @@
+{
+  "name": "engineering-conventions",
+  "owner": {
+    "name": "左林豪"
+  },
+  "description": "工程协作约定技能市场，提供通用 AI 编码协作约定。",
+  "plugins": [
+    {
+      "name": "engineering-conventions",
+      "source": "./",
+      "description": "通用工程协作约定：先找现成实现、公共代码改动先对齐、改实现同轮改文档、最小验证、长任务报进度、规范提交信息。",
+      "version": "1.1.0"
+    }
+  ]
+}