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/auto-build.sh

@@ -1,13 +1,23 @@
 #!/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.
+# Usage: auto-build.sh [--force]   (--force skips change detection — used
+# by the manual rebuild hook)
 set -eu
 
 REPO=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)
 VAULT="${VAULT:-/home/nox/docker/obsidian/vaults/weeslahw_coppermind}"
 STATE="$REPO/.build_state"
+FORCE=0
+[ "${1:-}" = "--force" ] && FORCE=1
 
 cd "$REPO"
+
+# serialize builds: the 5-min timer and the manual hook must never run
+# docker/npm into the same checkout concurrently
+exec 9>"$REPO/.build.lock"
+flock 9
+
 # a dirty tree can make pull fail; still rebuild whatever is checked out
 git pull -q || echo "git pull failed (dirty tree?) — building local state"
 
@@ -19,7 +29,7 @@ sig=$({
     find "$VAULT" -name .obsidian -prune -o -type f -printf '%T@ %p\n' | sort
 } | sha256sum | cut -d' ' -f1)
 
-if [ -f "$STATE" ] && [ "$(cat "$STATE")" = "$sig" ]; then
+if [ "$FORCE" -eq 0 ] && [ -f "$STATE" ] && [ "$(cat "$STATE")" = "$sig" ]; then
     exit 0
 fi