delete_entities tool (cascade delete by entity)

[JartanFTW]

Problem

Deleting all memories associated with an entity (e.g. a person, project, or concept) requires listing entities, finding all related memories, and deleting them individually. No atomic operation exists. delete_all is scope-based and doesn't map cleanly to entity-level cleanup.

Proposed Solution / Alternatives

A dedicated delete_entities tool that cascade-deletes all memories linked to a given entity. Cleaner semantic than overloading delete_all with entity-aware logic.