Retrieve Relevant Memories by Query

Implement retrieve(memories, query, scorer, top_k). - memories is a list of {"id": str, "content": str}. - scorer(query: str, content: str) -> float returns a relevance score. - Return the top_k memories sorted by score **descending**. - **Deterministic tie-breaking**: when two memories have the same score, prefer the one that appeared **first** in the original list (lower index). - Return a list of {"id": ..., "score": ...} dicts.