Stage 9: CI build on a self-hosted Gitea runner (0.7.0)

- .gitea/workflows/build.yml builds the packages and both ISOs with
  `make iso iso-server` on pushes to main and dev, v* tags and on demand
- ci/runner: setup of the CI host (WSL2 Ubuntu 24.04 with Docker Engine
  and gitea-runner as a systemd service, autostart task for Windows)
- scripts/builder.sh: DASOS_OUT keeps out/ outside the checkout, so CI
  reuses void-packages, the masterdir and the download cache
- README and TODO updated

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ci/runner/gitea-runner.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Gitea Actions runner for Das OS
+After=docker.service network-online.target
+Wants=network-online.target
+Requires=docker.service
+
+[Service]
+User=runner
+Group=runner
+WorkingDirectory=/var/lib/gitea-runner
+ExecStart=/usr/local/bin/gitea-runner -c /etc/gitea-runner/config.yaml daemon
+Restart=always
+RestartSec=10
+
+[Install]
+WantedBy=multi-user.target