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/auto-build.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# Timer target: rebuild the garden only when the vault or repo changed.
+# Successor to the Quartz update_quartz_docker.sh change-detection loop.
+set -eu
+
+REPO=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)
+VAULT="${VAULT:-/home/nox/docker/obsidian/vaults/weeslahw_coppermind}"
+STATE="$REPO/.build_state"
+
+cd "$REPO"
+git pull -q
+
+sig=$({
+    git rev-parse HEAD
+    find "$VAULT" -name .obsidian -prune -o -type f -printf '%T@ %p\n' | sort
+} | sha256sum | cut -d' ' -f1)
+
+if [ -f "$STATE" ] && [ "$(cat "$STATE")" = "$sig" ]; then
+    exit 0
+fi
+
+sh "$REPO/deploy/build.sh"
+printf %s "$sig" >"$STATE"
+echo "garden rebuilt $(date -Is)"