Spell the API host as api.${DOMAIN} in the worker and login examples

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013GAeg4Jo6xjZzjwy9QDWMQ

docs/code/workers.md

@@ -15,7 +15,7 @@ backend across hosts, which it never should.
 pip install fluksio-worker
 
 fluksio-worker \
-  --url wss://api.fluksio.com/api/v1/workers/attach \
+  --url wss://api.${DOMAIN}/api/v1/workers/attach \
   --token "$FLUKSIO_WORKER_TOKEN" \
   --labels gpu,cuda12 \
   --python /opt/torch-venv/bin/python
@@ -187,7 +187,7 @@ Write the clusters into `provisioners.json` beside the flows:
   "name": "hpc",
   "login": "me@login.cluster",
   "ssh_key": "/secrets/hpc_ed25519",
-  "engine_url": "wss://api.fluksio.com/api/v1/workers/attach",
+  "engine_url": "wss://api.${DOMAIN}/api/v1/workers/attach",
   "max_idle_s": 300,
   "provision_timeout_s": 900,
   "profiles": [{