I'm not sure I agree with this. I don't think there needs to be a whole spec & documentation process before plan mode.
There is alternative thought leadership that the waterfall approach for building out projects is not the right Agentic pattern[1].
Planning itself can be such an intensive process where you're designing and figuring out the specs on the fly in a focused manner for the thing the agent will actually develop next. Not sure how useful it is to go beyond this in terms of specs that live outside of the Agentic loop for what should be developed now and next.
I've evolved my own process, originally from plain Claude Code to Claude Code with heavy spec integrated capabilities. However, that became a burden for me: a lot of contextual drift in those documents and then self managing & orchestrating of Claude Code over those documents. I've since reoriented myself to base Claude Code with a fairly high-level effort specific to ad-hoc planning sessions. Sometimes the plans will revolve around specific GitHub issues or feature requests in the ticketing system, but that's about it.
[1] https://boristane.com/blog/the-software-development-lifecycl...