chore: bump version to 2.18.6

Update version and CHANGELOG for PR #303 test fix.

Fixed unit test failure in handleHealthCheck after implementing
environment-aware debugging improvements. Test now expects
troubleshooting array in error response details.

Changes:
- package.json: 2.18.5 → 2.18.6
- CHANGELOG.md: Added v2.18.6 entry with test fix details
- Comprehensive testing with n8n-mcp-tester agent confirms all
  environment-aware debugging features working correctly

🤖 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": "2.18.5",
+  "version": "2.18.6",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "bin": {