docs: add Phase 0 environment assessment and test baseline (#2)

Comprehensive assessment of core-admin package covering:
- Environment and dependency analysis
- Test baseline documentation (6 test files, 280+ tests)
- Architecture review (event-driven providers, 30 Livewire modals)
- Code quality status (strict typing, UK English conventions)
- Outstanding work prioritisation from TODO.md

Key findings:
- Package is healthy with excellent test coverage
- Cannot run standalone (requires parent Laravel app)
- Recent major testing push completed January 2026
- Recommended: Add PHPStan configuration for type safety

Co-Authored-By: Clotho <clotho@lthn.ai>

Author: Clotho