Show a skill and manage which people have it, at which of the skill's own proficiency levels.
Level selection is event-driven toggle chips (single-select skills replace,
multi-select toggle) rather than form checkboxes — an unchecked checkbox never
POSTs, so "deselect all" would be undetectable from form params. The add-form
stages its selection in @add_selected_levels; roster chips mutate the
assignment immediately via Skills.update_assignment_levels/2.