inspect: make model the primary slot edit, agent swap explicit

Editing a crew/judge slot previously re-prompted for the agent first, even though a slot's identity is its agent — so 'editing' the agent effectively replaced the slot, and the agent prompt was mislabeled 'Add a slot' on the edit path.

The per-slot menu now offers 'Change model' (the common case, a single step) and a distinct 'Change agent', the latter shown only when more than one candidate agent exists. Swapping the agent re-prompts the model since advertised models are agent-specific. Also drop the stale 'Labs entry' help assertion left over from trimming the inspect help text.