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>

Makefile

@@ -170,6 +170,7 @@ lint-backend:  ## Lint backend with ruff + mypy
 	cd backend && uv run ruff check .
 	cd backend && uv run ruff format --check .
 	cd backend && uv run mypy fluksio
+	cd worker && uv run --no-project --with mypy mypy fluksio_worker
 
 lint-frontend:  ## Lint frontend with biome
 	cd frontend && bun run lint