docs(memory): add cognitive memory extension pattern

[haroldfabla2-hue]

Summary

Adds a new section to the memory server documentation explaining how to extend the basic entity storage model into a full cognitive memory system with 4-tier architecture.

Motivation

The current memory server provides excellent entity/relation storage, but production AI agents often need more sophisticated memory architectures. This documentation helps developers understand:

• How the memory server fits into a larger cognitive memory system
• The 4-tier memory model (Working, Medium, Long-Term, Deep)
• Practical integration patterns with code example
• Reference to open-source implementations

Changes

• Added "🤖 Extending with Cognitive Memory Systems" section to
• Includes architecture comparison table
• Shows integration code pattern
• Links to silhouette-brain as reference implementation

Testing

Documentation only - no code changes.

Checklist

• Documentation added
• No breaking changes
• No new dependencies