Manual rebuild hook + polish: single title h1, '›' tree marker

- deploy/rebuild-hook.py: tiny stdlib HTTP service (:18101) — button
  page for the Homepage dashboard iframe, POST /rebuild runs
  auto-build.sh --force, /status + /healthz for feedback/monitoring
- auto-build.sh: --force flag + flock so timer and manual builds
  serialize
- [...slug].astro: skip the synthetic h1 when the note body already
  opens with an identical h1

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

deploy/garden-rebuild-hook.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Garden manual rebuild HTTP trigger (garden-hook.c0smere.net)
+After=network.target
+
+[Service]
+ExecStart=/usr/bin/python3 /home/nox/docker/garden-astro/deploy/rebuild-hook.py
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=default.target