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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.18.3",
+  "version": "2.18.4",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "bin": {