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/demo.env

@@ -12,7 +12,7 @@ DEMO_DOCKER_GID=996
 HOMEPAGE_PORT=4000
 DOCKER_SOCKET_PROXY_PORT=4005
 PIHOLE_PORT=4006
-PORTAINER_PORT=4007
+DOCKHAND_PORT=4007
 INFLUXDB_PORT=4008
 GRAFANA_PORT=4009
 DRAWIO_PORT=4010
@@ -28,7 +28,7 @@ ATUIN_PORT=4018
 DEMO_ADMIN_USER=admin
 DEMO_ADMIN_PASSWORD=demo_password
 DEMO_GRAFANA_ADMIN_PASSWORD=demo_password
-DEMO_PORTAINER_PASSWORD=demo_password
+DEMO_DOCKHAND_PASSWORD=demo_password
 
 # Network Configuration
 NETWORK_SUBNET=192.168.3.0/24