Reusable onboarding checklists

Admins can now create reusable onboarding checklists and assign them to any team. Checklists support per-step owners and due dates, and progress is visible from the team dashboard.

Provenance: 1/1 claims evidence-linked (100%) · content hash 613c754ec9af · release run 8283226a-b32b-4cb1-b883-cfe0d44358e0 — generated by shipsignal, human-approved at Gate #2.