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You can use the Added to Opportunity Trigger/Filter for this, with a constraint of Is Primary = True - e.g.,
One very unfortunate limitation is that the Is Primary constraint is NOT available on Opportunity Was Updated Trigger/Filter or Has Opportunity Filter.
So if someone gets added to an oppty and THEN gets changed to be primary, it would undetectable. (The same is true of the "Role" field.)
It beats me why contact role data is accessible in some places but not others.