chore(site): live-reload dev server via chokidar + browser-sync

Replaces the plain python3 http.server + one-shot make blog with a
watcher pipeline: chokidar regenerates HTML on MD/template changes,
browser-sync serves the site and reloads the browser on rendered-asset
changes. First run downloads both via npx; subsequent runs are instant.

Preflight checks for npx and pandoc. Port arg parsing is tolerant of
legacy --drafts flag ordering (drafts are always included now, since
that's what the dev loop actually wants).

Cleanup trap kills the watcher on exit so re-runs don't leave orphans.

scripts/serve-site.sh

@@ -1,14 +1,41 @@
 #!/usr/bin/env bash
+# Dev server for site/: regenerates drafts on each MD change, reloads the
+# browser on each rendered HTML/CSS/JS change. Port is the first numeric arg
+# (default 9000); any other args are ignored for back-compat.
+#
+# First run downloads chokidar-cli + browser-sync into the npm cache — slow
+# once, instant after that.
+
 set -euo pipefail
 
-PORT="${1:-9000}"
-
-if [[ "${1:-}" == "--drafts" ]] || [[ "${2:-}" == "--drafts" ]]; then
-  PORT="${PORT//--drafts/9000}"  # default port if --drafts was first arg
-  make blog-drafts
-else
-  make blog
+PORT=9000
+for arg in "$@"; do
+  if [[ "$arg" =~ ^[0-9]+$ ]]; then
+    PORT="$arg"
+    break
   fi
+done
 
-echo "Serving site at http://localhost:$PORT"
-cd site && python3 -m http.server "$PORT"
+command -v npx >/dev/null || { echo "npx not found. Install Node.js: https://nodejs.org" >&2; exit 1; }
+command -v pandoc >/dev/null || { echo "pandoc not found (required by 'make blog-drafts')." >&2; exit 1; }
+
+# Initial render so the first page load has everything.
+make blog-drafts
+
+echo "Serving site at http://localhost:$PORT (drafts included, live reload)"
+
+# Kill child processes on exit so re-runs don't leave orphaned watchers.
+trap 'kill $(jobs -p) 2>/dev/null' EXIT INT TERM
+
+# Regenerate HTML when MD sources or the blog template change.
+npx --yes chokidar-cli \
+  "drafts/*.md" "blog/*.md" "site/blog-template.html" \
+  -c "make blog-drafts" &
+
+# Serve + reload on rendered-asset changes.
+cd site && exec npx --yes browser-sync start \
+  --server . \
+  --port "$PORT" \
+  --files "**/*.html,**/*.css,**/*.js" \
+  --no-open \
+  --no-notify