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 <noreply@anthropic.com>

deploy/garden-astro-build.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=Periodic garden rebuild check (every 5 min, matches old Quartz cadence)
+
+[Timer]
+OnCalendar=*:0/5
+Persistent=true
+
+[Install]
+WantedBy=timers.target