curve-symmetry-lean: the coding agents' handoff file leaves the public tree

curve-symmetry-lean no longer carries ONBOARDING.md, the coding agents’ handoff and tracker, removed at the author’s request (d99bc17); it stays in the git history, last version at f618c4a, and the README says so for the older verification notes that cite it. The README now lists the repository’s tags through sprint-4-complete and sprint-7-complete, and STATUS records the Sprint 7 commit’s two CI runs. This is the follow-up to the 30 September publication, not a new result.