fix(store): error on embedding dimension mismatch instead of silent rebuild (#501)

When switching to an embedding model with different dimensions,
ensureVecTableInternal() silently drops the vector table and all
embeddings are lost. Users only discover this when semantic search
returns empty results.

Throw an error instead, telling users to run 'qmd embed -f' to
explicitly re-embed. This is safe because embed -f calls
clearAllEmbeddings() which drops the table before ensureVecTable
is reached.

Related to #497

Co-authored-by: JohnRichardEnders <john@telli.com>

src/store.ts

@@ -1053,7 +1053,12 @@ function ensureVecTableInternal(db: Database, dimensions: number): void {
     const hasCosine = tableInfo.sql.includes('distance_metric=cosine');
     const existingDims = match?.[1] ? parseInt(match[1], 10) : null;
     if (existingDims === dimensions && hasHashSeq && hasCosine) return;
-    // Table exists but wrong schema - need to rebuild
+    if (existingDims !== null && existingDims !== dimensions) {
+      throw new Error(
+        `Embedding dimension mismatch: existing vectors are ${existingDims}d but the current model produces ${dimensions}d. ` +
+        `Run 'qmd embed -f' to re-embed with the new model.`
+      );
+    }
     db.exec("DROP TABLE IF EXISTS vectors_vec");
   }
   db.exec(`CREATE VIRTUAL TABLE vectors_vec USING vec0(hash_seq TEXT PRIMARY KEY, embedding float[${dimensions}] distance_metric=cosine)`);