Add Hugo upgrade script

bin/upgrade

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+build_file="$root_dir/.build.yml"
+
+if ! command -v hugo >/dev/null 2>&1; then
+  echo "hugo not found in PATH" >&2
+  exit 1
+fi
+
+hugo_version_raw="$(hugo version)"
+hugo_version="$(printf '%s\n' "$hugo_version_raw" | sed -E 's/^hugo v?([0-9]+\.[0-9]+\.[0-9]+).*/\1/')"
+if [[ -z "$hugo_version" || "$hugo_version" == "$hugo_version_raw" ]]; then
+  echo "Unable to parse hugo version from: $hugo_version_raw" >&2
+  exit 1
+fi
+
+current_version="$(rg -n "HUGO_VERSION:" "$build_file" | sed -E 's/.*HUGO_VERSION:[[:space:]]*//')"
+if [[ -z "$current_version" ]]; then
+  echo "Unable to read HUGO_VERSION from $build_file" >&2
+  exit 1
+fi
+
+if [[ "$hugo_version" == "$current_version" ]]; then
+  echo "Hugo already at $hugo_version"
+  exit 0
+fi
+
+python3 - <<PY
+from pathlib import Path
+
+path = Path("$build_file")
+data = path.read_text()
+old = "$current_version"
+new = "$hugo_version"
+needle = f"HUGO_VERSION: {old}"
+if needle not in data:
+    raise SystemExit("Expected HUGO_VERSION not found in .build.yml")
+path.write_text(data.replace(needle, f"HUGO_VERSION: {new}", 1))
+PY
+
+git add "$build_file"
+git commit -m "Upgrade Hugo to $hugo_version"
+
+echo "Updated HUGO_VERSION from $current_version to $hugo_version"