BREAHK CHANGING: save rag in YAML instead of bin

Here are the reasons:

1. Bincode cannot extend fields. In feat: support RAG-scoped rag_top_k and rag_reranker_model #847, two fields, reranker_model and top_k, were added to the RAG struct. When loading the RAG structure from the bin format, the following error occurs.

Error: Failed to load rag 'aichat-wiki' at '<aichat-config-dir>/rags/aichat-wiki.bin'

Caused by:
    tag for enum is not valid, found 32

2. Bincode cannot handle data structures like serde_json::Value.

3. YAML is a text format that allows users to view rag details, such as chunk content, embedding data, etc. Make it easier for us to debug or troubleshoot.