docs: add production service architecture guide to public docs

Added comprehensive production service architecture guide with singleton patterns, caching strategies, and performance optimization for Express/Node.js services.

Changes:
- NEW: docs/PRODUCTION_SERVICE_ARCHITECTURE.md - Complete guide for using Brainy in production services
- CHANGED: .gitignore - Removed PRODUCTION_*.md pattern to allow public documentation
- CHANGED: README.md - Added subtle link to production architecture guide in "Production MVP" section

Guide covers:
- Instance-per-request anti-pattern (40x memory waste)
- Singleton pattern implementation (40x memory reduction, 30x faster)
- Three implementation patterns (simple, service class, middleware)
- Optimization strategies (cache sizing, lazy loading, warm-up)
- Concurrency and thread safety
- Production checklist and monitoring

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -197,6 +197,7 @@ const brain = new Brainy({
 })
 ```
 **Scale:** Thousands to hundreds of thousands • **Performance:** <5ms queries
+**→ [Production Service Architecture](docs/PRODUCTION_SERVICE_ARCHITECTURE.md)** — Singleton patterns, caching, and scaling for Express/Node.js services
 
 ### 🏢 Growing Company → Multi-Million Scale
 ```javascript