feat: complete Phase 1-4 - Production-ready RAG package with full test coverage

[thaolaptrinh]

Summary

Complete implementation of a production-ready, provider-agnostic RAG package for Laravel with 4 phases:

• Phase 1: Core abstractions (7 contracts, 5 exceptions, TextDataSource, TextChunker, database migration)
• Phase 2: Ingestion pipeline (OpenAIEmbeddingDriver, PgVectorStore, IngestionPipeline, rag:ingest command)
• Phase 3: Query + generation pipeline (SimilarityRetriever, SimplePromptBuilder, OpenAILlmDriver, QueryPipeline, rag:query command)
• Phase 4: Laravel integration (ServiceProvider, Facade, config, comprehensive tests)

Changes in this PR

• Fix facade binding by introducing RagManager as the 'rag' singleton
• Add --topK option to rag:query command
• Remove skeleton boilerplate migration stub
• Refactor arch tests to Pest syntax with expanded coverage (5 arch tests)
• Add 10 integration tests (service provider bindings, facade delegation, E2E data flow)

Test Plan

• 46 tests pass (36 unit + 10 integration)
• 144 assertions
• PHPStan level 6+ — 0 errors
• Architecture tests verify no provider lock-in
• All files have declare(strict_types=1);
• Constructor dependencies use readonly

Quality Checklist

• ✅ Provider-agnostic architecture (no OpenAI/Pinecone in services)
• ✅ Batch operations (embedBatch, storeMany)
• ✅ Idempotent ingestion (duplicate-safe inserts)
• ✅ Structured logging with trace IDs
• ✅ Custom exceptions for error handling
• ✅ JSON_THROW_ON_ERROR for all JSON decoding