feat(skill): 新增 app-install-manager 应用安装管理技能

补齐委派式安装的执行半环：核心 Agent 收到安装/卸载/启停指令后，读 registry
install.md、跑 docker compose、健康校验，并把真实结果回写 installed-entries.json
状态（installing 转 installed/failed），后端 watcher 据此自动派生/清理服务。

- skills/app-install-manager/SKILL.md（procedural / risk:high，含安装/卸载/启停 SOP）
- agent.json: default_enabled.skills 加该技能；version 1.6.2 转 1.7.0
- skills/_index.md 补索引

agent.json

@@ -2,7 +2,7 @@
   "name": "DesireCore",
   "$schema": "http://desirecore/schemas/agent.json",
   "id": "7da73b7f-bb08-4e7b-a3cf-5d4af6e22c7f",
-  "version": "1.6.2",
+  "version": "1.7.0",
   "description": "系统中枢调度器，负责任务分发、Agent 编排与全局状态监控",
   "author": "DesireCore Team",
   "repository": {
@@ -19,7 +19,8 @@
       "delete-agent",
       "self-evolve",
       "task-management",
-      "discover-agent"
+      "discover-agent",
+      "app-install-manager"
     ],
     "tools": []
   },