fix: implement unified UUID-based sharding for metadata across all storage adapters

Fixes critical scalability bottleneck where metadata was stored in non-sharded
directories, causing performance degradation at scale (1M+ entities).

Changes:
- Add UUID-based sharding to metadata operations in S3Compatible, FileSystem, and OpFS storage
- Implement complete UUID-based sharding for OpFS storage (nouns, verbs, metadata)
- Update pagination methods to iterate through all 256 UUID-based shards
- Add integration tests verifying sharding behavior across storage adapters

Impact:
- Metadata now scales to millions of entities without directory bottlenecks
- All storage adapters now use consistent UUID-based sharding (256 buckets: 00-ff)
- Improves GCS/S3/R2/OpFS performance at scale
- Path format: entities/{type}/{subtype}/{shard}/{id}.json

Breaking change: Requires data migration for existing S3/GCS/R2/OpFS deployments.
See .strategy/UNIFIED-UUID-SHARDING.md for migration guidance.

src/storage/adapters/fileSystemStorage.ts

@@ -627,7 +627,13 @@ export class FileSystemStorage extends BaseStorage {
   protected async saveNounMetadata_internal(id: string, metadata: any): Promise<void> {
     await this.ensureInitialized()
 
-    const filePath = path.join(this.nounMetadataDir, `${id}.json`)
+    // Use UUID-based sharding for metadata (consistent with noun vectors)
+    const filePath = this.getShardedPath(this.nounMetadataDir, id)
+
+    // Ensure shard directory exists
+    const shardDir = path.dirname(filePath)
+    await fs.promises.mkdir(shardDir, { recursive: true })
+
     await fs.promises.writeFile(filePath, JSON.stringify(metadata, null, 2))
   }
 
@@ -637,7 +643,8 @@ export class FileSystemStorage extends BaseStorage {
   public async getNounMetadata(id: string): Promise<any | null> {
     await this.ensureInitialized()
 
-    const filePath = path.join(this.nounMetadataDir, `${id}.json`)
+    // Use UUID-based sharding for metadata (consistent with noun vectors)
+    const filePath = this.getShardedPath(this.nounMetadataDir, id)
     try {
       const data = await fs.promises.readFile(filePath, 'utf-8')
       return JSON.parse(data)