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/docker-compose.yml

@@ -1,8 +1,8 @@
 # Serves the prebuilt dist/ — run deploy/build.sh first.
 services:
-  garden-next:
+  garden:
     image: nginx:alpine
-    container_name: garden-next
+    container_name: garden
     restart: unless-stopped
     ports:
       - "18100:80"