feat(retrieve): add keyword retriever option and per-call override

[jl1371-cell]

What

• Add RetrieveConfig.retriever option for RAG item recall: vector / keyword.
• Support per-call override: MemoryService.retrieve(..., method=..., retriever=...).
• Support wrapper-level default retriever in the OpenAI wrapper (set once at init).

Why

Keyword retriever provides a lightweight & explainable fallback, and enables deterministic matching when embeddings are unavailable.

Behavior (no breaking changes)

• Default behavior unchanged: if users don’t pass retriever, RAG continues to use the default (vector).
• Precedence:
  • per-call retriever (if provided) overrides RetrieveConfig.retriever
  • per-call method (if provided) overrides RetrieveConfig.method
• Wrapper compatibility: when retriever is None, the wrapper does not forward retriever downstream (avoids errors if downstream service doesn’t accept this arg).

Implementation

• RetrieveConfig gains retriever (normalized case-insensitively).
• RetrieveMixin.retrieve accepts optional method/retriever; writes state["retriever"] for RAG workflows.
• _rag_recall_items chooses keyword/vector based on state["retriever"].
• OpenAI wrapper threads an optional default retriever; per-call still overrides.

Tests

• make test
• make check

Notes

• Tests that require external API keys are conditionally skipped when related env vars are not set (e.g. OPENAI_API_KEY, OPENROUTER_API_KEY).