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.

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

demo/docs/troubleshooting/README.md

@@ -121,18 +121,18 @@ docker compose logs grafana
 # Navigate to: http://localhost:4009/datasources
 ```
 
-#### Portainer Container Access
+#### Dockhand Container Access
 **Symptoms**: Can't manage containers
 **Solution**:
 ```bash
-# Check Docker socket proxy
-docker compose logs docker-socket-proxy
+# Check Dockhand logs
+docker compose logs dockhand
 
-# Verify proxy permissions
-curl http://localhost:4005/version
+# Verify Docker socket access
+docker exec tsysdevstack-supportstack-demo-dockhand docker version
 
-# Restart Portainer
-docker compose restart portainer
+# Restart Dockhand
+docker compose restart dockhand
 ```
 
 ### Performance Issues