I created this using PLANS.md and it basically replicates a kanban/scrum process with gated approvals per stage, locked artifacts when it moves to next stage, etc. It works very well and it doesnt need a UI. Sure, I could have several agents running at the same time, but I believe manual QA is key to keeping the codebase clean, so time spent on this today means that future requirements can be implemented 10x faster than with a messy codebase.
But what is your concept of "stages"? For me, the spec files are a MECE decomposition, each file is responsible for its unique silo (one file owns repo layout, etc), with cross references between them if needed to eliminate redundancy. There's no hierarchy between them. But I'm open to new approaches.