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>

src/index.ts

@@ -760,7 +760,6 @@ export class CodeGraph {
     if (!this.vectorManager) {
       this.vectorManager = createVectorManager(this.db.getDb(), this.queries, {
         embedder: {
-          cacheDir: path.join(this.projectRoot, '.codegraph', 'models'),
           showProgress: true,
         },
       });