docs: [#28] add multi-provider architecture implementation plan

This plan implements a clean multi-provider architecture that properly separates
environments from infrastructure providers, ensuring the system can scale to support
unlimited providers without code changes.

Key design principles:
- Clear separation: Environment vs Provider (development/staging/production vs libvirt/hetzner/aws)
- Pluggable provider system with standard interface functions
- Scalable architecture requiring zero code changes for new providers
- Zero breaking changes with backward compatibility

Implementation phases:
1. Foundation - Rename environments, create provider interface
2. Provider System - Move libvirt to provider module, create Hetzner provider
3. Enhanced Commands - Update Makefile to require ENVIRONMENT + PROVIDER
4. Hetzner Implementation - Complete Hetzner Cloud provider
5. Testing and Documentation

Addresses parent issue #3 Phase 4: Hetzner Infrastructure Implementation

Author: josecelano