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/main.py

@@ -8,6 +8,7 @@ from fastapi import FastAPI
 from fastapi.concurrency import run_in_threadpool
 from fastapi.responses import JSONResponse
 from fastapi.routing import APIRoute
+from fluksio_worker.worker_main import ARTIFACT_DIR_ENV
 from starlette.middleware.cors import CORSMiddleware
 
 from fluksio.api.main import api_router
@@ -33,7 +34,6 @@ from fluksio.flow.secrets import init_secrets
 from fluksio.flow.state import MemoryState, RedisState, StateBackend
 from fluksio.flow.store import FlowStore
 from fluksio.flow.watchdog import LoopWatchdog
-from fluksio.flow.worker_main import ARTIFACT_DIR_ENV
 from fluksio.flow.workers import PythonWorkerPool