fix: correct contrast pattern in anti-AI table

'不是...就是...' means 'either...or...' (disjunction), not the
intended contrast. Changed to '不是...只是...' which preserves
the original 'not X, but merely Y' meaning while still being
more natural than '并非...而只是...'.

Addresses Copilot and Codex review feedback.

docs/zh/llm-prompt.md

@@ -23,7 +23,7 @@ Write like a developer explaining something to a colleague over lunch — clear,
 | 将其复制到...文件中 | 复制到...里 | "将其" is literary, not spoken |
 | 该行显示了...的URL地址 | 这行告诉你...的地址 | "该" as a pronoun is stiff; "地址" already implies URL |
 | 你将会看到 | 你会看到 | "将会" is news-anchor tone |
-| 它并非...而只是... | 不是...就是... | "并非...而只是" is essay-style |
+| 它并非...而只是... | 不是...只是... | "并非...而只是" is essay-style; "不是...只是..." preserves the contrast |
 | 可能像下面这样 | 长这样 / 像这样 | "可能" is unnecessary hedging |
 | 由于路径首先匹配，因此始终会使用第一个定义的 | 路径优先匹配，永远走第一个 | Remove "由于...因此" scaffolding |
 | 通过从 str 继承，API 文档就能... | 继承 str 后，API 文档会... | "通过从...继承" is English grammar mapped to Chinese |