build: add scripts/build-zip.sh to package plugin as a Decky-installable ZIP

The ZIP bundles plugin.json, main.py, package.json, dist/index.js (+ map) at
the archive root (no enclosing folder) — the format Decky's 'Install Plugin
from ZIP File' / 'Install Plugin from URL' expects.

Usage:
  bun run build:zip   # build + package → releases/plugin-v<version>.zip
  bun run zip         # package already-built dist only

plugins/decky-vault/scripts/build-zip.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+# Build a Decky Loader plugin ZIP ready for "Install Plugin from ZIP File".
+#
+# The ZIP must contain the plugin files at the ROOT (no enclosing folder):
+#   plugin.json, main.py, package.json, dist/index.js [, dist/index.js.map]
+#
+# Usage:
+#   ./scripts/build-zip.sh          # builds + packages, output in releases/
+#   ./scripts/build-zip.sh --no-build  # package only (skip rollup build)
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+VERSION=$(node -p "require('./package.json').version")
+NAME=$(node -p "require('./package.json').name")
+# Strip any @scope/ prefix for the zip filename
+ZIP_NAME=$(echo "$NAME" | sed 's#.*/##')
+
+OUT_DIR="releases"
+ZIP_PATH="$OUT_DIR/${ZIP_NAME}-v${VERSION}.zip"
+
+if [[ "${1:-}" != "--no-build" ]]; then
+  echo "› Building plugin with rollup…"
+  bun run build
+fi
+
+echo "› Packaging $ZIP_PATH …"
+rm -rf "$OUT_DIR"
+mkdir -p "$OUT_DIR"
+
+# Create the zip with files at the root. We cd into the plugin dir so paths
+# are relative (no enclosing folder in the archive).
+zip -r -X "$PWD/$ZIP_PATH" \
+  plugin.json \
+  main.py \
+  package.json \
+  dist/index.js \
+  dist/index.js.map \
+  >/dev/null
+
+echo "✓ Built $ZIP_PATH ($(du -h "$ZIP_PATH" | cut -f1))"
+echo
+echo "Install options:"
+echo "  • Decky → Plugin Browser → ⋮ → Install Plugin from ZIP File → pick $ZIP_PATH"
+echo "  • Or host $ZIP_PATH anywhere and use 'Install Plugin from URL'"