feat(agents): Phase 7-10 hook system, plugins, skills, orchestration

Phase 7: Built-in Hooks (audit_log, dangerous_confirmation, security_scan)
Phase 8: Plugin system (PluginManager, PluginSandbox, PluginManifest)
Phase 9: Skills registry (SkillRegistry, local/plugin/MCP loaders)
Phase 10: TeamLeader, RemoteTransport, BackgroundTaskManager

backend/app/agents/plugins/__init__.py

@@ -0,0 +1,12 @@
+"""插件系统 - Phase 8"""
+
+from app.agents.plugins.manager import PluginManager, get_plugin_manager
+from app.agents.plugins.manifest import PluginManifest
+from app.agents.plugins.sandbox import PluginSandbox
+
+__all__ = [
+    "PluginManager",
+    "PluginManifest",
+    "PluginSandbox",
+    "get_plugin_manager",
+]