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-test.sh

@@ -180,7 +180,7 @@ test_port_accessibility() {
         "$HOMEPAGE_PORT:Homepage"
         "$DOCKER_SOCKET_PROXY_PORT:Docker Socket Proxy"
         "$PIHOLE_PORT:Pi-hole"
-        "$PORTAINER_PORT:Portainer"
+        "$DOCKHAND_PORT:Dockhand"
         "$INFLUXDB_PORT:InfluxDB"
         "$GRAFANA_PORT:Grafana"
         "$DRAWIO_PORT:Draw.io"