ci: pull-based deploy to the Pi via rolling dev release

The Pi is on a closed travel-router LAN, so push-based deploy from a
runner can't reach it. Switch to pull: the runner builds + publishes,
the Pi fetches.

- build-dev.yaml: after the arm64 build, publish the binary + sha256 +
  version.txt to a rolling "dev" Gitea release (replaces the
  upload-artifact + repository_dispatch -> deploy-dev hop)
- remove deploy-dev.yaml (push/scp-based deploy no longer used)
- scripts/pi-update.sh: poll the dev release, verify sha256, install via
  deploy.sh (backup/restart/rollback); only updates when version changes
- scripts/remoterig-update.{service,timer}: run the updater every 5 min
- setup-pi.sh: install deploy.sh + pi-update.sh + update.env template +
  the updater timer; summary now reflects the pull flow
- README: document the pull-based CI/CD; fix stale GOARM=6 (Zero 2 W is
  arm64 on 64-bit OS / arm GOARM=7 on 32-bit)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/remoterig-update.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Periodically check for RemoteRig updates (Gitea dev release)
+
+[Timer]
+OnBootSec=2min
+OnUnitActiveSec=5min
+Persistent=true
+
+[Install]
+WantedBy=timers.target