Final answer:
Lincoln's support for gradual compensated emancipation in the Border States best supports the writer's idea that "statesmanship is not an abstract skill, but a contextual one".
Step-by-step explanation:
The detail that best supports the writer's idea that "statesmanship is not an abstract skill, but a contextual one" is Lincoln's support for gradual compensated emancipation in the Border States. The passage states that Lincoln believed voluntary acceptance of emancipation would have better long-term results than a forced arrangement. This detail shows that Lincoln understood the need to consider the specific context and circumstances of the Border States in order to reconcile them back to the Union.