Add default_search_type to config

[calebjpicker]

Currently the default search type is hardcoded: hybrid when semantic search is enabled, text otherwise (_default_search_type() in mcp/tools/search.py).

It would be useful to have a default_search_type option in .basic-memory/config.json so users can set their preferred default (e.g., vector) without specifying search_type on every query.

Use case: I'm using OpenAI's large embedding model locally and want to default to pure vector search to take full advantage of those embeddings, while still being able to override per-query when needed.