chore: add prepare script so the package builds on git install

Enables consuming the package as a git dependency (e.g. from Gitea): npm runs
'prepare' on install, which builds dist/ (git-ignored).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -44,6 +44,7 @@
   },
   "scripts": {
     "build": "tsup",
+    "prepare": "npm run build",
     "example": "npm run build && node --env-file=examples/.env examples/demo.mjs",
     "test": "vitest run",
     "test:watch": "vitest",