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>

README.md

@@ -35,7 +35,7 @@ GARDEN_CONTENT=/path/to/vault/Digital_Garden npm run build
 Defaults to the kotov vault copy. On cyrion use `deploy/build.sh`
 (containerized node:24, vault mounted read-only), then
 `docker compose -f deploy/docker-compose.yml up -d` serves `dist/` on
-port 18100 (`garden-next.c0smere.net` while staging).
+port 18100, proxied as `garden.c0smere.net` (NPM public + Caddy internal).
 
 ## Layout extras