Implement Layer 4: Relationship Inference System

Layer 4 provides relationship inference by analyzing memory patterns
and user personality. This is an optional layer used when game or
companion features are active.

Core Philosophy:
- Independent from Layers 1-3.5 (optional feature)
- Inference-based, not stored (computed on-demand)
- Simple calculation using existing data
- Foundation for external applications (games, companions, etc.)

Implementation (src/core/relationship.rs):
- RelationshipInference struct with key metrics:
  * interaction_count: number of memories with entity
  * avg_priority: average priority of those memories
  * days_since_last: recency of interaction
  * bond_strength: inferred strength (0.0-1.0)
  * relationship_type: close_friend, friend, acquaintance, etc.
  * confidence: data quality indicator (0.0-1.0)

Inference Logic:
- Personality-aware: introverts favor count, extroverts favor quality
- Simple rules: bond_strength from interaction patterns
- Automatic type classification based on metrics
- Confidence increases with more data

MCP Tools (src/mcp/base.rs):
- get_relationship(entity_id): Get specific relationship
- list_relationships(limit): List all relationships sorted by strength

Design Decisions:
- No caching: compute on-demand for simplicity
- No persistence: relationships are derived, not stored
- Leverages Layer 1 (related_entities) and Layer 3.5 (profile)
- Can be extended later with caching if needed

Usage Pattern:
- Normal use: Layers 1-3.5 only
- Game/Companion mode: Enable Layer 4 tools
- Frontend calls get_relationship() for character interactions
- aigpt backend provides inference, frontend handles presentation

src/core/mod.rs

@@ -2,10 +2,12 @@ pub mod analysis;
 pub mod error;
 pub mod memory;
 pub mod profile;
+pub mod relationship;
 pub mod store;
 
 pub use analysis::UserAnalysis;
 pub use error::{MemoryError, Result};
 pub use memory::Memory;
 pub use profile::{UserProfile, TraitScore};
+pub use relationship::{RelationshipInference, infer_all_relationships};
 pub use store::MemoryStore;