Restore effect:identity executor shortcut

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

legacy_tasks.py

@@ -94,8 +94,15 @@ class DogExecutor(Executor):
         return dog_process(inputs, output_path, config, None)
 
 
-# Note: No specific executor for identity - let the generic EFFECT executor
-# handle it so effects can be loaded from IPFS by CID when specified.
+@register_executor("effect:identity")
+class IdentityExecutor(Executor):
+    """Executor for the identity effect (passthrough)."""
+
+    def execute(self, config: Dict, inputs: List[Path], output_path: Path) -> Path:
+        from artdag.nodes.effect import effect_identity
+        if len(inputs) != 1:
+            raise ValueError(f"Identity effect expects 1 input, got {len(inputs)}")
+        return effect_identity(inputs[0], output_path, config)
 
 
 @register_executor(NodeType.SOURCE)