docs site: deploy from a dedicated clean clone; poll against last deployed commit

- docs-deploy-poll.service and the receiver now build from
  /home/ubuntu/.docs-publisher, so uncommitted edits in a working clone can
  never block a deploy
- the poller compares origin/main against the last DEPLOYED commit (state
  file), so a push made from the deploy clone itself also triggers a deploy

publishing/docs-deploy-poll.service

@@ -26,8 +26,9 @@ Wants=network-online.target
 
 [Service]
 User=ubuntu
-WorkingDirectory=/home/ubuntu/repos/docs
-ExecStart=/bin/bash /home/ubuntu/repos/docs/publishing/poll-watcher.sh
+WorkingDirectory=/home/ubuntu/.docs-publisher
+Environment=DOCS_REPO_DIR=/home/ubuntu/.docs-publisher
+ExecStart=/bin/bash /home/ubuntu/.docs-publisher/publishing/poll-watcher.sh
 Restart=on-failure
 RestartSec=10