I've tried this, and it is largely what I do, but it's easy to end up with a large tangled pile of markdown files, and the implementation either diverges from what's in markdown or you burn a lot of tokens keeping them in-sync.

The plans usually end up very long and verbose, and in my most vibe-y test project I stopped even reading them myself and instead just ask questions interactively. e.g. "what are the remaining unanswered design decisions", "which part is jankiest". It does work quite well although I don't know how I feel about it.

P.S. to any anthropic staff: The "chat about this" option in Plan mode is useful but also annoying. When I select it I want to take the next turn of the conversation myself, I don't want it to waste time "thinking" just for it to verbosely ask me what I want to chat about.