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📖 Refactor sampleexternalplugin to be a valid reference implementation

Transform the sample external plugin from mock scaffolding to a realistic
Prometheus monitoring generator that demonstrates best practices for external
plugin development.

Key changes:
- Implement init and edit subcommands that scaffold Prometheus instance manifests
- Add PROJECT config reading to align with internal plugin patterns
- Replace mock ServiceMonitor with complete Prometheus CR including RBAC
- Add --namespace flag support to demonstrate argument passing
- Update documentation with realistic examples showing argument usage
- Fix error handling to properly validate PROJECT file existence
- Centralize kustomization in config/default/kustomization.yaml (no separate prometheus kustomization)
- Provide clear instructions for users to add Prometheus resources manually

The plugin now serves as a proper reference implementation for:
- Reading PROJECT configuration in external plugins
- Scaffolding production-ready Kubernetes manifests
- Supporting command-line arguments (--namespace flag)
- Testing plugins against local Kubebuilder source
- Adding optional monitoring features via edit subcommand
- Following Kubebuilder's centralized kustomization pattern

Addresses maintainer feedback on PR 5116 including proper error handling,
argument passing examples, realistic manifest generation, and centralized kustomization.