feat: User-defined memory layers + domain isolation

[prakashUXtech]

Summary

The protocol should not dictate memory structure. Layers should be user-defined namespaces, not a hardcoded enum. Domains should isolate context within layers.

Right now MemoryType is a fixed enum: CORE, EPISODIC, SEMANTIC, PROCEDURAL. The brainstorm vision says the SDK should give you soul.layer("procedural").store(...) where layer names are arbitrary.

Spec Changes (spec/)

• Replace fixed MemoryType with open string-based layer names
• Add domain field to MemoryEntry (optional namespace like "finance", "legal", "personal")
• MemoryStore protocol accepts layer + domain as query parameters
• .soul file format: memory/{layer_name}/{domain}/ directory structure

Runtime Changes (runtime/)

• Default layers: core, episodic, semantic, procedural (backward compatible)
• New social layer for relationship memory (bonds, trust, interaction history)
• Domain-aware MemoryManager — queries can scope to a domain
• DomainIsolationMiddleware — enforces that agents only access their authorized domains

Why This Matters

• Makes the protocol truly general-purpose, not tied to our 4-layer opinion
• Domain isolation is Cognee's insight — agents in billing shouldn't see legal context
• Social layer enables multi-agent relationship memory
• Aligns with "HTTP not nginx" — the spec defines the namespace mechanism, not the names

Migration

• MemoryType enum stays in runtime/ as a convenience, but spec/ uses plain strings
• Existing .soul files still work (default domain = "default")

Sizing: L