docs: add reindex note for v0.19 upgrade

Tells local users to run `bm reindex --embeddings` after upgrading to
generate vector embeddings for semantic search.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phernandez

content/2.whats-new/0.v0.19.0.md

@@ -269,6 +269,10 @@ Semantic search is now on out of the box. Embeddings are generated automatically
 | Embedding generation | Manual | Automatic on first startup |
 | Install size | Smaller (no embedding deps) | Larger (includes fastembed) |
 
+::note
+After upgrading to v0.19, run `bm reindex --embeddings` to generate vector embeddings for semantic search. This is a one-time operation for local projects. Cloud projects handle embedding generation automatically.
+::
+
 **Restore old behavior:** Set `semantic_search_enabled: false` in config, or set `BASIC_MEMORY_SEMANTIC_SEARCH_ENABLED=false`.
 
 ### Frontmatter added on sync