添加 AI 辅助创作标注组件和贡献指南说明

[Copilot]

AI 生成内容需要明确告知读者。在贡献指南中增加 AI 辅助创作的使用规范，并提供对应的前端展示组件。

Changes

• content/contributing.smd: 新增"AI 辅助创作说明"章节，说明使用规范及注意事项；在 frontmatter 示例中添加 ai_generated 字段示例
• layouts/post.shtml: 当 custom.ai_generated = true 时，在文章末尾渲染提示横幅
• assets/style.css: 新增 .ai-notice 样式，支持亮色/暗色模式

Usage

在需要标注 AI 辅助内容的文章 frontmatter 中添加：

.custom = {
  .ai_generated = true,
},

文章末尾将自动显示：

⚠️ 本文包含 AI 辅助生成的内容，请结合自身判断阅读，作者对内容准确性负责。

[Copilot]

Pull request overview

This PR adds a standardized way to disclose AI-assisted content on the site: it documents the frontmatter flag for authors and renders a visible notice banner on posts when that flag is enabled.

Changes:

• Documented an ai_generated frontmatter flag in the contributing guide, including usage guidelines.
• Rendered an AI-assistance notice banner at the end of posts when custom.ai_generated = true.
• Added CSS styling for the new notice banner with light/dark mode support.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
content/contributing.smd	Adds contributor guidance and examples for marking AI-assisted articles via frontmatter.
layouts/post.shtml	Conditionally renders an AI-assistance notice banner based on custom.ai_generated.
assets/style.css	Introduces .ai-notice styles for the banner in light/dark mode.

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[Copilot]

This notice banner will be included in the Pagefind indexed body (it sits inside data-pagefind-body and does not have data-pagefind-ignore), which can pollute search excerpts with the same boilerplate warning on every AI-marked post. Consider adding data-pagefind-ignore to the .ai-notice container (similar to #prev-next, #meta, and TOC blocks) so search indexing focuses on the article content.

Suggested change

	<div class="ai-notice" :if="$page.custom.getOr('ai_generated', false)">
	<div class="ai-notice" :if="$page.custom.getOr('ai_generated', false)" data-pagefind-ignore>

[Copilot]

The new .ai-notice styling introduces a separate yellow/orange palette via hard-coded hex values, even though the stylesheet already defines shared design tokens (e.g. --brand-accent, --bg-*, --text-*) for light/dark theming. To keep the theme consistent and avoid duplicating color decisions, consider reusing existing tokens (or adding dedicated --ai-notice-* tokens in :root + dark override) instead of embedding raw hex colors in the component rules.