inspect: drop per-inspector model selection from guided setup

Inspectors run on the agent's default model, so the interactive crew flow no longer asks for or edits a per-inspector model:

• adding a slot picks only the agent,
• the slot-edit menu offers Change agent (when >1 candidate) + Remove, no Change model,
• the slot label shows a model only when one was set explicitly (e.g. via scripted --set-slot agent=model, which still works).

The judge keeps its model picker (model matters for the consolidating call). Scripted/runtime model handling is unchanged.