Combine agent + model selection into one review-crew screen

Replace the guided setup's multi-step flow (which agents? → choose models? → per-agent model → add-variant loop) with a single "Build the review crew" screen: one multiselect per launchable agent listing its models plus Default and Custom…. Checking multiple models under an agent creates that many workers (same agent, different models) — making the variant concept native instead of a hidden loop. Default is pre-checked, so pressing enter yields one worker per agent on its default model.

• Add promptForReviewCrew + resolveCrewModel; remove promptForSimpleReviewAgents, promptForSimpleReviewModels, promptForModelChoice, agentNamesToTypes.
• Workers always set cfg.Agent, so prompt/model-only agents (e.g. Pi) are now selectable in guided setup — previously skipped because their default config is empty.