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/README.md

@@ -0,0 +1,21 @@
+# fluksio-worker
+
+Runs [Fluksio](https://fluksio.com) nodes on a machine the engine cannot reach —
+a GPU box, a cluster node, anything behind a NAT. It dials *out* to the engine
+over one authenticated websocket, so nothing has to be exposed here.
+
+```sh
+pip install fluksio-worker
+
+fluksio-worker \
+  --url wss://api.example.com/api/v1/workers/attach \
+  --token "$FLUKSIO_WORKER_TOKEN" \
+  --labels gpu,cuda12 \
+  --python /opt/torch-venv/bin/python
+```
+
+`--python` is the interpreter node code runs on, which is how this machine keeps
+its own wheels without the engine ever installing them. Mint a token from the
+engine with `POST /api/v1/workers/tokens`.
+
+Linux and macOS.