feat: Phase 3 插件与工具系统 — Plugin SDK + Plugin Manager + 13内置插件 (40文件, 3293行)

- Plugin SDK: Plugin/Tool/ComplexTool/HostAPI 标准化接口
- Plugin Manager: 插件生命周期管理 (Install/Enable/Disable/Uninstall/Reload)
- Tool Registry: 聚合工具注册表 (Register/Execute/Dispatch)
- 13 个内置插件: 将原有硬编码工具迁移为标准插件格式
- REST API: 11 个端点 (net/http, 零外部依赖)
- ai-core 集成: PluginManagerClient 替代本地工具调用
- plugin.json 元数据: 每个插件含完整 author/version/category/permissions

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/plugin-manager/plugins/file/plugin.json

@@ -0,0 +1,12 @@
+{
+  "name": "file",
+  "displayName": "File Operations",
+  "version": "1.0.0",
+  "minCyreneVersion": "1.0.0",
+  "author": { "name": "Cyrene Team" },
+  "description": "Sandboxed file operations: read, write, list, delete within DATA_DIR",
+  "license": "MIT",
+  "keywords": ["file", "read", "write"],
+  "category": "system",
+  "permissions": ["file:read", "file:write"]
+}