Engagements: proposals → agreements module

The engagements module (proposals, acceptance, agreements) — slices 1–2 implemented with the engagements-lifecycle E2E green in CI; contract still Draft (approval tracked under launch hardening). Acceptance v2 (sign + pay ceremony) is its own Planned card and lands on this module. Remaining in-module: lifecycle polish and the accepting state machinery that acceptance v2 introduces.