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>

src/layouts/Base.astro

@@ -455,7 +455,7 @@ const showToc = headings.filter((h) => h.depth <= 3).length >= 2;
         list-style: none;
       }
       .sidebar summary::before {
-        content: '▸';
+        content: '›';
         display: inline-block;
         width: 0.9em;
         color: var(--muted);