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

.gitignore

@@ -50,3 +50,6 @@ yarn-error.log*
 # stray
 post.md
 plugins/decky-vault/dist/
+
+# Decky plugin release zips
+plugins/decky-vault/releases/