Update RAG based on file changes

[tomsib2001]

I love the new RAG feature! I use it to ask questions to my notes. However, sometimes my notes change, and I would love if there was a way for aichat to detect changes and reindex only the portions which have changed.

[sigoden]

AIChat will not support updating RAG when files changes, but it provides a .rebuild rag command to synchronize changes manually (#672).

AIChat RAG supports not only local files and directories, but also remote files and websites, you cannot simply watch them.