Add delete(origin) method to BaseStore

[dfalbel]

Summary

BaseStore currently has no way to delete a document by origin. This is a basic CRUD gap — we have create/read/update (upsert, retrieve) but no delete.

Proposed API

# In BaseStore
@abstractmethod
def delete(self, origin: str) -> bool:
    """Delete a document and its chunks by origin.
    
    Returns True if the document was found and deleted, False if not found.
    """
    ...

Each backend implements it:

• DuckDBStore: delete from documents + chunks tables by origin
• ChromaDBStore: delete matching IDs from the collection
• OpenAIStore: find and delete the file from the vector store