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/package.json

@@ -5,7 +5,9 @@
   "type": "module",
   "scripts": {
     "build": "rollup -c",
-    "watch": "rollup -c -w"
+    "watch": "rollup -c -w",
+    "build:zip": "rollup -c && ./scripts/build-zip.sh --no-build",
+    "zip": "./scripts/build-zip.sh"
   },
   "dependencies": {
     "@decky/api": "^1.1.3",