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>

.gitignore

@@ -3,3 +3,5 @@ dist/
 .astro/
 public/assets/
 .build_state
+.build.lock
+.rebuild.log