Make review crew slot-based (arbitrary workers, incl. duplicate agent+model)

Reframe crew configuration around worker slots rather than "per agent". A slot is an agent + model; you can define any number, including the same agent multiple times on different OR identical models (e.g. 5 Claude slots). The data model already supported this (profile.Agents is keyed by an arbitrary worker name, disambiguated by workerIDForAgentModel) — this change updates the two config surfaces that were still agent-keyed.

Picker: replace the per-agent model grid with a slot list — seeded with one default-model slot per launchable agent, then add/remove/duplicate slots (each add = pick agent → pick model/Default/Custom).

Scripted: add repeatable --set-slot agent[=model]. --set-agents stays as the one-default-slot-per-agent shorthand (keeps --set-model targeting). Duplicate slots are allowed and become distinct workers.