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.

💘 Generated with Crush

Assisted-by: GLM-4.7 via Crush <crush@charm.land>

demo/scripts/demo-stack.sh

@@ -184,7 +184,7 @@ display_summary() {
     echo "📊 Homepage Dashboard: http://localhost:${HOMEPAGE_PORT:-4000}"
     echo "🏗️ Infrastructure Services:"
     echo "   - Pi-hole (DNS): http://localhost:${PIHOLE_PORT:-4006}"
-    echo "   - Portainer (Containers): http://localhost:${PORTAINER_PORT:-4007}"
+    echo "   - Dockhand (Containers): http://localhost:${DOCKHAND_PORT:-4007}"
     echo "📊 Monitoring & Observability:"
     echo "   - InfluxDB (Database): http://localhost:${INFLUXDB_PORT:-4008}"
     echo "   - Grafana (Visualization): http://localhost:${GRAFANA_PORT:-4009}"