fix: resolve sql.js adapter bypass in NodeRepository constructor (Issue #296)

Changes duck typing ('db' in object) to instanceof check for precise type discrimination.
Only unwraps SQLiteStorageService instances, preserving DatabaseAdapter wrappers intact.

Fixes MCP tool failures (get_node_essentials, get_node_info, validate_node_operation)
on systems using sql.js fallback (Node.js version mismatches, ARM architectures).

- Changed: NodeRepository constructor to use instanceof SQLiteStorageService
- Fixed: sql.js queries now flow through SQLJSAdapter wrapper properly
- Impact: Empty object returns eliminated, proper data normalization restored

Closes #296

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/database/node-repository.ts

@@ -7,11 +7,12 @@ export class NodeRepository {
   private db: DatabaseAdapter;
   
   constructor(dbOrService: DatabaseAdapter | SQLiteStorageService) {
-    if ('db' in dbOrService) {
+    if (dbOrService instanceof SQLiteStorageService) {
       this.db = dbOrService.db;
-    } else {
-      this.db = dbOrService;
+      return;
     }
+
+    this.db = dbOrService;
   }
   
   /**