Production cutover: rename container to garden, add auto-rebuild timer

- deploy/auto-build.sh: git pull + rebuild only when the repo HEAD or any
  vault file mtime changed (sha256 signature in .build_state); replaces
  the Quartz update_quartz_docker.sh loop
- systemd user units (5-min cadence, matching the old Quartz timer)
- compose service/container renamed garden-next -> garden

Co-Authored-By: Claude Opus 4.8 <[email protected]>

deploy/garden-astro-build.service

@@ -0,0 +1,6 @@
+[Unit]
+Description=Rebuild Astro garden when the vault or repo changed
+
+[Service]
+Type=oneshot
+ExecStart=/home/nox/docker/garden-astro/deploy/auto-build.sh