Add prepare script for npx GitHub install

When npx installs from GitHub, it runs the prepare script to build.
Without this, dist/ doesn't exist and the bin entry fails.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -9,6 +9,7 @@
     "vice-mcp": "dist/index.js"
   },
   "scripts": {
+    "prepare": "npm run build",
     "build": "tsup src/index.ts --format esm --dts --clean",
     "dev": "tsup src/index.ts --format esm --watch",
     "start": "node dist/index.js",