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/fluksio/flow/workers.py

@@ -26,11 +26,15 @@ from collections.abc import Callable
 from pathlib import Path
 from typing import Any
 
+from fluksio_worker import worker_main as _worker_main
+
 from fluksio.flow.events import EventBus
 
 logger = logging.getLogger(__name__)
 
-WORKER_MAIN = Path(__file__).with_name("worker_main.py")
+#: The runner, in the worker distribution — the same file a remote worker runs,
+#: so a node cannot tell which kind of worker it is on.
+WORKER_MAIN = Path(_worker_main.__file__)
 
 #: Importing what a node needs can be slow the first time; compiling is not
 #: something a person is watching a spinner for.