review: RunMulti falls back to cfg.Model like Run

Run() falls back to cfg.Model when a reviewer carries no model metadata, but RunMulti only read reviewerModelName(r), leaving AgentRun.Model empty for reviewers that don't implement reviewerRunMetadata — which would drop them from session-to-manifest matching. Mirror Run's fallback in RunMulti.

In the current wiring the multi reviewers are perAgentConfiguredReviewer (which implements the metadata interface), so this is a defensive consistency fix rather than a live break, but it makes the two orchestrators behave identically. Adds a test via stubReviewer (no model metadata) asserting the fallback to cfg.Model.