~x-ai/grok-latest → A (3:1)

A fixes a real multi-user blocker: shared WEB_BROWSER_AGENT sentinels bound on first browser vote and rejected later humans; making PendingSession.agent optional and posting UI votes with no delegate restores correct multi-user behavior and is backed by a full integration test. B is a correct but narrower reducer fix that only reorders the zero-ratio early-return so ensure_item/voted_pairs do not leave ghosts—valuable hygiene, smaller lasting scope than A.