Split the worker into a distribution of its own

A cluster or GPU host installs `pip install fluksio-worker` and gets the
agent and the runner, not psycopg, numpy and the MCP SDK. The engine
depends on it as a workspace member, so the file it launches node code
with is the same file a remote worker runs — which is what keeps a node
unable to tell the difference. Copying the two files by hand still works.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/flow/test_workers.py

@@ -6,10 +6,10 @@ import time
 from collections.abc import Iterator
 
 import pytest
+from fluksio_worker.worker_main import ARTIFACT_DIR_ENV
 
 from fluksio.flow.artifacts import ArtifactStore
 from fluksio.flow.messages import DType, MessageSpec
-from fluksio.flow.worker_main import ARTIFACT_DIR_ENV
 from fluksio.flow.workers import NodeCancelled, NodeTimeout, PythonWorkerPool