fix: resolve 10 test failures across clustering, metadata, and deletion

Fixed critical bugs affecting test suite:

**Clustering (2 tests fixed)**
- Fixed entity.type field reference bug in _getItemsByField()
- Changed entity.noun to entity.type (correct Entity interface field)
- Now includes ALL entities in domain clustering with 'unknown' fallback

**Relationship Metadata (5 tests fixed)**
- Fixed metadata retrieval in memoryStorage.ts getVerbs()
- Changed metadata.data to metadata.metadata for user's custom metadata
- User metadata now correctly returned in GraphVerb.metadata field

**Delete Relationship Cleanup (2 tests fixed)**
- Added deleteVerbMetadata() method to BaseStorage
- Fixed deleteVerb_internal() in memoryStorage to delete verb metadata
- Relationships now properly cleaned up when entities are deleted

**Validation (1 test fixed)**
- Removed overly restrictive self-referential relationship check
- Self-relationships now allowed (valid in graph systems)

Test results: 27 failures → 17 failures (37% improvement)
All 467 tests now enabled (0 skipped)

src/utils/paramValidation.ts

@@ -239,11 +239,10 @@ export function validateRelateParams(params: RelateParams): void {
   if (!params.to) {
     throw new Error('to entity ID is required')
   }
-  
-  if (params.from === params.to) {
-    throw new Error('cannot create self-referential relationship')
-  }
-  
+
+  // Allow self-referential relationships - they're valid in graph systems
+  // (e.g., a person can be related to themselves, a file can reference itself, etc.)
+
   // Validate verb type - default to RelatedTo if not specified
   if (params.type === undefined) {
     params.type = VerbType.RelatedTo