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>

worker/fluksio_worker/__main__.py

@@ -0,0 +1,5 @@
+"""``python -m fluksio_worker`` — the same entry point as the console script."""
+
+from fluksio_worker.agent import main
+
+raise SystemExit(main())