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/string_sha256.py

@@ -0,0 +1,45 @@
+"""Workflow plugin: compute SHA256 hash of string/bytes."""
+
+import hashlib
+from ...base import NodeExecutor
+
+
+class StringSha256(NodeExecutor):
+    """Compute SHA256 hash of input string or bytes."""
+
+    node_type = "string.sha256"
+    category = "string"
+    description = "Compute SHA256 hash of input string or bytes"
+
+    def execute(self, inputs, runtime=None):
+        """
+        Compute SHA256 hash.
+
+        Args:
+            inputs: Dict with:
+                - input: String or bytes to hash
+                - prefix: Optional bool, whether to prepend "sha256:" (default: False)
+
+        Returns:
+            Dict with 'result' containing hex hash string
+        """
+        input_value = inputs.get("input", "")
+        prefix = inputs.get("prefix", False)
+
+        # Convert to bytes if string
+        if isinstance(input_value, str):
+            input_bytes = input_value.encode('utf-8')
+        else:
+            input_bytes = input_value
+
+        # Compute hash
+        hash_obj = hashlib.sha256(input_bytes)
+        hex_hash = hash_obj.hexdigest()
+
+        # Add prefix if requested
+        if prefix:
+            result = f"sha256:{hex_hash}"
+        else:
+            result = hex_hash
+
+        return {"result": result}