Add diff preview updates for suitup config reruns

[Copilot]

Summary

• Add safe line-preserving merge/diff support for suitup-managed config updates.
• Preview unified diffs and require confirmation before applying shipped additions.
• Preserve user-added config lines and skip unmarked or prompt/theme files with documented reasons.
• Add BDD-style Vitest coverage and update English/Chinese docs.

Validation

• npm test passes (211 tests).
• Parallel validation: Code Review passed; CodeQL passed once, then timed out on the final rerun with an instruction not to rerun.

[ChangeHow]

@copilot curl 的方式没有办法触发增量修改的模式吗？我不是增加了后台加载 completion 的能力吗？我重新通过 curl 运行 setup 步骤的时候并没有更新到这个能力。