chore(versioning): reposition project as 0.9.0 (pre-1.0)

docs(reference): remove sprint narrative (Phase 2 L3)

test(rename): dé-sprintage tests/reports (29 fichiers, git mv)

docs(sprint-H.9): archive migration plans + cleanup stale doc paths

docs: refonte Diataxis + 8 documents institutionnels (S60)