The "ideal critical thinker" is described as a habitual disposition. What kind of personality trait is critical thinking on this view?

A habitus — a habitual stance toward facts, data and theories marked by accuracy, care and circumspection.

On this framing critical thinking is less a one-off act than a personality trait (Persönlichkeitsmerkmal) — part of a habitus, a settled, habitual way of dealing with facts, data and theories, characterised by accuracy, care and circumspection.

The classic Facione portrait (1990) spells out the habits: the ideal critical thinker is habitually inquisitive, well-informed, reason-oriented, flexible, fair-minded in evaluation, honest about personal bias, careful in judgement, willing to reconsider, clear about intentions, systematic with complex matters, diligent in seeking information, reasonable in choosing criteria, focused in inquiry, and persistent in seeking results as precise as the subject allows.

Tip: The repeated word is habitually. The point is that critical thinking is a disposition you cultivate until it's automatic — not a checklist you remember to consult.