docs(claude): restructure documentation management and add feedback system

- Restructure §8 from "文档沉淀规则" to "文档管理规则" with 4 subsections
  - Add docs/ structure with features/ and knowledge-base/ directories
  - Add feature documentation template with 7 sections (概述/设计初衷/技术设计/预期作用/实际效果/演化路线/头脑风暴)
  - Add feature update trigger matrix (新增/修改/完成/问题/反馈)
  - Add documentation quality checklist
- Add §16

desktop/src/lib/viking-client.ts

@@ -327,3 +327,26 @@ export class VikingError extends Error {
     this.name = 'VikingError';
   }
 }
+
+// === Singleton ===
+
+let _instance: VikingHttpClient | null = null;
+
+/**
+ * Get the singleton VikingHttpClient instance.
+ * Uses default configuration (localhost:1933).
+ */
+export function getVikingClient(baseUrl?: string): VikingHttpClient {
+  if (!_instance) {
+    _instance = new VikingHttpClient(baseUrl);
+  }
+  return _instance;
+}
+
+/**
+ * Reset the singleton instance.
+ * Useful for testing or reconfiguration.
+ */
+export function resetVikingClient(): void {
+  _instance = null;
+}