feat: ID-first storage architecture + remove memory-unsafe APIs (v6.0.0)

BREAKING CHANGES:

**ID-First Storage Paths**
- Direct O(1) entity access without type lookups
- Before: entities/nouns/{TYPE}/metadata/{SHARD}/{ID}.json
- After:  entities/nouns/{SHARD}/{ID}/metadata.json
- Migration handled automatically on first init()

**Removed Memory-Unsafe APIs**
- Removed brain.merge() - loaded all entities into memory
- Removed brain.diff() - loaded all entities into memory
- Removed brain.data().backup() - loaded all entities into memory
- Removed brain.data().restore() - depended on backup()
- Removed CLI commands: backup, restore, cow merge

**Migration Paths**
- merge() → Use checkout() or manually copy entities with pagination
- diff() → Use asOf() with manual paginated comparison
- backup() → Use fork() for instant COW snapshots
- restore() → Use checkout() to switch to snapshot branch

Core Improvements:
- ✅ All 8 storage adapters properly call super.init()
- ✅ GraphAdjacencyIndex integration in BaseStorage.init()
- ✅ Fixed ID-first path bugs (vector.json → vectors.json)
- ✅ Fixed MemoryStorage.initializeCounts() for ID-first paths
- ✅ New VFS APIs: du(), access(), find()
- ✅ Comprehensive documentation with migration guides

Storage Adapters Fixed:
- MemoryStorage, FileSystemStorage, AzureBlobStorage
- GCSStorage, R2Storage, S3CompatibleStorage
- OPFSStorage, HistoricalStorageAdapter

Files Changed: 28 files, +1,075/-1,933 lines (net -858)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/vfs/vfs-bug-fixes.unit.test.ts

@@ -234,19 +234,19 @@ describe('VFS Bug Fixes', () => {
       await vfs.writeFile('/src/b.ts', 'b')
       await vfs.writeFile('/docs/README.md', 'readme')
 
-      // Get statistics
-      const stats = await vfs.getProjectStats('/')
+      // Get statistics using du() (standard POSIX API)
+      const stats = await vfs.du('/')
 
       // Debug: Check what directories exist
       const rootChildren = await vfs.getDirectChildren('/')
       console.log('Root children (stats test):', rootChildren.map(c => ({name: c.metadata.name, type: c.metadata.vfsType})))
 
       // Should have exactly 3 files
-      expect(stats.fileCount).toBe(3)
+      expect(stats.files).toBe(3)
 
       // Should have exactly 2 directories (src, docs)
       // Note: If there's a duplicate, this will fail
-      expect(stats.directoryCount).toBe(2)
+      expect(stats.directories).toBe(2)
     })
   })
 })