fix: extract first line as plain-text title from Javadoc comments

[shalousun]

• Add DocUtil.getCommentFirstLine() to extract first line with HTML tags stripped, producing a clean plain-text title for headings
• Update all template builders to use getCommentFirstLine for desc field instead of the full Javadoc comment
• Fix detail fallback to use full comment when desc is now truncated
• Conditionally render Description section in all HTML/Markdown templates — hidden when doc.detail is empty (no comment written)

[github-actions]

@shalousun

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