openai/gpt-chat-latest → B (10:1)

Side B makes a functional change to sibling navigation by treating each unranked sibling as its own navigation group instead of lumping all unranked items together, updates the implementation accordingly, and adds a regression test verifying the new grouping behavior. Side A only deletes planning documents (`plan.md` and `plan2.md`), removing design notes without changing the project's runtime behavior.