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)

tests/unit/neural/domain-time-clustering.test.ts

@@ -22,7 +22,7 @@ describe('Domain and Time Clustering', () => {
   })
 
   describe('clusterByDomain() - Field-based clustering', () => {
-    it.skip('should cluster entities by type field', async () => {
+    it('should cluster entities by type field', async () => {
       // Add entities of different types
       await brain.add(createAddParams({
         data: 'John Smith is a person',
@@ -99,7 +99,7 @@ describe('Domain and Time Clustering', () => {
       expect(domains.has('cooking')).toBe(true)
     })
 
-    it.skip('should handle entities without the specified field', async () => {
+    it('should handle entities without the specified field', async () => {
       // Add entities with and without category
       await brain.add(createAddParams({
         data: 'Has category',