refactor(demo): replace Portainer with Dockhand

Replace Portainer container management service with Dockhand:
- Update docker-compose.yml.template with Dockhand service definition
- Replace portainer_data volume with dockhand_data
- Update PORTAINER_PORT to DOCKHAND_PORT in demo.env
- Update all script references (demo-stack.sh, demo-test.sh, validate-all.sh)
- Update integration test from Portainer to Dockhand
- Update documentation files (README.md, AGENTS.md, api-docs,
  service-guides, troubleshooting)

Dockhand provides modern Docker management UI with:
- Container lifecycle management
- Compose stack orchestration
- Git-based deployments
- Multi-environment support
- Terminal access and log streaming
- File browser capabilities

Maintains same port (4007) for consistency.

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AGENTS.md

@@ -34,7 +34,7 @@ TSYSDevStack-SupportStack-LocalWorkstation/
 │   │   ├── homepage/            # Homepage dashboard config
 │   │   ├── grafana/             # Grafana dashboards/datasources
 │   │   ├── pihole/              # Pi-hole configuration
-│   │   ├── portainer/           # Portainer configuration
+│   │   ├── dockhand/            # Dockhand configuration
 │   │   ├── influxdb/            # InfluxDB configuration
 │   │   ├── drawio/              # Draw.io configuration
 │   │   ├── kroki/               # Kroki configuration