feat(plugin): add plugin interface system and iot-mock implementation

- internal/plugin: Plugin/IoTProvider/ToolProvider/RAGProvider interfaces
- internal/plugin: Registry with type-safe auto-detection, 11 tests
- plugins/iot-mock: 8 simulated smart home devices, 11 tests
- 63 total tests, build + vet clean

backend/internal/plugin/tool.go

@@ -0,0 +1,27 @@
+package plugin
+
+import "context"
+
+// ToolProvider is implemented by plugins that provide LLM-callable tools.
+// Multiple ToolProviders can be active simultaneously.
+type ToolProvider interface {
+	Plugin
+
+	// Tools returns the list of tool definitions this plugin provides.
+	Tools() []ToolDefinition
+
+	// Execute invokes a tool by name with the given parameters.
+	// Returns the result string or an error.
+	Execute(ctx context.Context, toolName string, params map[string]any) (string, error)
+}
+
+// RAGProvider is implemented by plugins that provide RAG (Retrieval-Augmented Generation).
+type RAGProvider interface {
+	Plugin
+
+	// Ingest adds documents to the knowledge base.
+	Ingest(ctx context.Context, docs []RAGDocument) error
+
+	// Search performs a semantic search over the knowledge base.
+	Search(ctx context.Context, query string, topK int) ([]RAGSearchResult, error)
+}