feat(workflow): add packagerepo and string.sha256 plugins

Created 11 packagerepo-specific workflow plugins:
- auth_verify_jwt - JWT token verification
- auth_check_scopes - Scope-based authorization
- parse_path - URL path parameter extraction (Express-style)
- normalize_entity - Field normalization (trim, lower, unique, sort)
- validate_entity - JSON schema validation
- kv_get/kv_put - RocksDB key-value operations
- blob_put - Filesystem blob storage with SHA-256 hashing
- index_upsert - Index entry management
- respond_json/respond_error - Response formatting

Created string.sha256 plugin:
- Compute SHA256 hash of strings/bytes
- Optional "sha256:" prefix
- Used by packagerepo for content-addressed storage

All plugins follow standard pattern:
- Class extending NodeExecutor
- Factory with create() function
- package.json with metadata
- Access external state via runtime parameter

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

workflow/plugins/python/string/string_sha256/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "@metabuilder/string_sha256",
+  "version": "1.0.0",
+  "description": "Compute SHA256 hash of input string or bytes",
+  "author": "MetaBuilder",
+  "license": "MIT",
+  "keywords": ["string", "workflow", "plugin", "hash", "sha256", "crypto"],
+  "main": "string_sha256.py",
+  "files": ["string_sha256.py", "factory.py"],
+  "metadata": {
+    "plugin_type": "string.sha256",
+    "category": "string",
+    "class": "StringSha256",
+    "entrypoint": "execute"
+  }
+}