feat: 插件-工具合并 — 创建 pkg/plugins 共享模块并移除 tool-engine

- 新增 backend/pkg/plugins/ 共享模块：SDK 接口、PluginManager、ToolRegistry（含环形缓冲区调用日志）
- 13 个通用插件从 plugin-manager 迁移至共享模块（import 路径统一）
- ai-core 切换至共享 ToolRegistry，进程内执行（零网络开销），包装 6 个专属工具
- plugin-manager 迁移至共享模块，保留管理 REST API
- 新增 DevTools 插件管理面板（侧边栏 → 🔌 插件管理）
- 移除 tool-engine 服务（从 go.work、DevTools 配置、编译系统）
- 工具调用记录 API 从 Tool-Engine 迁至 AI-Core（/api/v1/tools/calls）
- ai-core ContextStore 启动时从 PostgreSQL 恢复会话历史
- 清理所有过时引用和备份文件

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

backend/pkg/plugins/sdk/plugin.go

@@ -0,0 +1,49 @@
+package sdk
+
+import (
+	"context"
+	"net/http"
+)
+
+// Plugin is the main interface every plugin must implement.
+type Plugin interface {
+	Metadata() PluginMetadata
+	Init(ctx context.Context, config PluginConfig) error
+	Start(ctx context.Context, host HostAPI) error
+	Stop(ctx context.Context) error
+	Health(ctx context.Context) error
+	Tools() []Tool
+}
+
+// Tool is the interface every tool must implement.
+type Tool interface {
+	Definition() ToolDefinition
+	Execute(ctx context.Context, args map[string]interface{}) (*ToolResult, error)
+	Validate(args map[string]interface{}) error
+	Complexity() ToolComplexity
+}
+
+// ComplexTool extends Tool for async multi-round execution.
+type ComplexTool interface {
+	Tool
+	ExecuteAsync(ctx context.Context, args map[string]interface{}) (<-chan ToolProgress, error)
+	Cancel(ctx context.Context, executionID string) error
+}
+
+// HostAPI gives plugins access to Cyrene core capabilities.
+type HostAPI interface {
+	CallLLM(ctx context.Context, messages []LLMMessage) (*LLMResponse, error)
+	SearchMemory(ctx context.Context, userID, query string, limit int) ([]MemoryEntry, error)
+	StoreMemory(ctx context.Context, entry MemoryEntry) error
+	Logger() Logger
+	GetConfig(key string) (string, error)
+	SetConfig(key, value string) error
+	PublishEvent(ctx context.Context, event map[string]interface{}) error
+	HTTPClient() *http.Client
+}
+
+// Logger is a minimal logging interface for plugins.
+type Logger interface {
+	Printf(format string, args ...interface{})
+	Println(args ...interface{})
+}