Download embedding model to ~/.codegraph/models on install

The nomic-ai model is now downloaded during npm install via a postinstall
script and stored globally in ~/.codegraph/models (shared across projects)
instead of per-project in .codegraph/models.

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

package.json

@@ -9,10 +9,12 @@
   },
   "files": [
     "dist",
+    "scripts",
     "README.md"
   ],
   "scripts": {
     "build": "tsc && npm run copy-assets",
+    "postinstall": "node scripts/postinstall.js",
     "copy-assets": "cp -r src/extraction/queries dist/extraction/ && cp src/db/schema.sql dist/db/",
     "dev": "tsc --watch",
     "cli": "npm run build && node dist/bin/codegraph.js",