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/docs/service-guides/README.md

@@ -18,7 +18,7 @@ All services are accessible through the Homepage dashboard at http://localhost:4
 
 ### 🏗️ Infrastructure Services
 - **Pi-hole** (Port 4006): DNS management with ad blocking
-- **Portainer** (Port 4007): Web-based container management
+- **Dockhand** (Port 4007): Modern Docker management UI
 - **Docker Socket Proxy** (Port 4005): Secure Docker socket access
 
 ### 📊 Monitoring & Observability
@@ -45,7 +45,7 @@ All services are accessible through the Homepage dashboard at http://localhost:4
 - **Username**: `admin`
 - **Password**: `demo_password`
 
-These credentials work for Grafana and Portainer. Other services may have different authentication requirements.
+These credentials work for Grafana and Dockhand. Other services may have different authentication requirements.
 
 ## Getting Help