feat(graph): wire the proposals read to service.ListProposals (Phase 3)

The Proposals port declared a proposal listing and left Options.Proposals
nil, so the `proposals` query failed loudly with "service/ exposes no
proposal listing yet". Phase 3 supplies it: an adapter maps
service.Proposal onto graph.Proposal at the edge — the two structs are
identical, but service/ must not import graph/, so the rename lives here
beside web.NewReader's equivalent — and cmd wires graph.NewProposals(svc)
into the schema. The `proposals` field now answers from service/.