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/README.md

@@ -95,7 +95,7 @@ All configuration is managed through `demo.env` and dynamic detection:
 | Service | Port | Description | 🌐 Access |
 |---------|------|-------------|-----------|
 | **Pi-hole** | 4006 | DNS-based ad blocking and monitoring | [Open](http://192.168.3.6:4006) |
-| **Portainer** | 4007 | Web-based container management | [Open](http://192.168.3.6:4007) |
+| **Dockhand** | 4007 | Modern Docker management UI | [Open](http://192.168.3.6:4007) |
 
 ### 📊 Monitoring & Observability
 | Service | Port | Description | 🌐 Access |