Fix compilation errors in Layer 4 caching

- Remove unused RelationshipInference import from base.rs
- Add Parse error variant to MemoryError enum
- Replace num_minutes() with num_seconds() / 60 (chrono compatibility)

Fixes compatibility with different chrono versions.

src/core/error.rs

@@ -19,6 +19,9 @@ pub enum MemoryError {
 
     #[error("Configuration error: {0}")]
     Config(String),
+
+    #[error("Parse error: {0}")]
+    Parse(String),
 }
 
 pub type Result<T> = std::result::Result<T, MemoryError>;

src/core/store.rs

@@ -530,7 +530,7 @@ impl MemoryStore {
                     .map_err(|e| MemoryError::Parse(e.to_string()))?
                     .with_timezone(&Utc);
 
-                let age_minutes = (Utc::now() - cached_at).num_minutes();
+                let age_minutes = (Utc::now() - cached_at).num_seconds() / 60;
 
                 if age_minutes < Self::RELATIONSHIP_CACHE_DURATION_MINUTES {
                     let relationship: super::relationship::RelationshipInference =
@@ -582,7 +582,7 @@ impl MemoryStore {
                     .map_err(|e| MemoryError::Parse(e.to_string()))?
                     .with_timezone(&Utc);
 
-                let age_minutes = (Utc::now() - cached_at).num_minutes();
+                let age_minutes = (Utc::now() - cached_at).num_seconds() / 60;
 
                 if age_minutes < Self::RELATIONSHIP_CACHE_DURATION_MINUTES {
                     let relationships: Vec<super::relationship::RelationshipInference> =

src/mcp/base.rs

@@ -2,7 +2,7 @@ use anyhow::Result;
 use serde_json::{json, Value};
 use std::io::{self, BufRead, Write};
 
-use crate::core::{Memory, MemoryStore, UserAnalysis, RelationshipInference, infer_all_relationships, get_relationship};
+use crate::core::{Memory, MemoryStore, UserAnalysis, infer_all_relationships, get_relationship};
 
 pub struct BaseMCPServer {
     store: MemoryStore,